Hopefully you see the fundamental idea -- We can basically convert any D&D d20-based target number to d6 by dividing by 3 and rounding down (20/3 ~ 6.67), and this allows us to roll lots of 1:10 figure attacks with big fistfuls of d6's (as in many other wargames). You can easily convert any D&D monsters to this system by using the exact same Movement and Hit Dice values, and a single conversion for the "Armor Hit" target value on d6 (that being eminently memorable). At this point, I've done it on the fly several times for large numbers of D&D random wilderness encounters, and it's worked very well.
Any other modifiers from D&D can likewise get converted in a divide-by-3 operation (noting that many minor adjustments will disappear from our abstraction entirely). In fact, if you use new-style "ascending Armor Class", then you can just go ahead and divide that by 3 for your AH, as well (for some time I was working on this game under 3E principles, and while that part worked, the rest of the 3E system was pretty much too complicated to deal with -- which was one reason among many that I switched back to OD&D, around the time this blog started).
More than one person has told me now that they've actually played some games of man-to-man D&D itself using this cut-down d6 system, with good results (although that wasn't the original intent).
Now, let's pose some questions about whether the scales chosen make sense. I suppose the first assumption is simply what kind of miniature figures we'll be using: and I definitely wanted to use the "standard" 25-28mm figures that you've already got for your D&D or Warhammer games, with base sizes as noted above (those being my best guess for what they're already on). With that as a given:
Why 10 men per figure?
Having thought about this quite a bit, I came to the exact same conclusion that Gygax did in the Introduction to Swords & Spells. I wound up re-discovering every point that he made there:
After considerable contemplation a 10:1 ratio was decided upon. If this seems somewhat small for a supposedly large-scale set of rules, the following factors must be remembered: First, most fantasy battles which involve numbers too large to handle at 1:1 are still on a relatively small scale -- hundreds and thousands rather than tens of thousands. More importantly, the exceptional creatures had to be allowed for, and this could not practically be done on a scale greater than 10:1. [S&S, p. 1]Indeed. If mass figures are at 1:10 scale (10 HD per figure), then a 10+ level monster or character can appear as an equivalent-strength independent figure (plus, it's in the tradition of works like Swords & Spells and Battlesystem). If mass scale was 1:20 (as in Chainmail), then that would effectively rule out any hero figures of less than 20th level -- notably, all the monsters in Original D&D.
Why 20 feet per inch?
This took considerably more thought. Here are 3 separate pieces of evidence why this is the correct scale:
(1) We can do a direct math calculation. If Chainmail is correct that 1:20 mass action works at 1"=30 feet (and I think it does, from a realism perspective), then by halving the men per figure, the overall area covered should also be halved. Say the original area is given on the order of A=s2 (s is the old scale in feet, i.e., 30), that is, s=√A. Then our new scale should be s'=√(A/2). Compute: s'=√(A/2)=√A/√2 = s/√2 ~ 30/1.4 = 21 feet. Round that off to 20 feet for convenience.
(2) We can reason from the size of individual men in the figure. Assume that the 10 men in each figure are arrayed in 2 ranks of 5 men each (that's an important assumption!), and that they take up 3 feet each, shoulder-to-shoulder in formation (historical double-check; 168/56=3). Then compute our resulting scale from that and the figure base size: 3 feet/man × 5 men wide/figure × 1 figure/(0.75 inch) = 20 feet/inch.
(3) Consider scale ships & castles. (This was an important consideration for my games -- in fact, the original motivation was largely to be able to operate a D&D naval campaign in a reasonable way.) If you make historical ships & castles at a scale of 1"=30 feet, then they're actually too small for the figures we've chosen to stand on top of them. But if you use 1"=20 feet, i.e., larger scale models, then our figures fit beautifully on top of realistic boats, tower-tops, gatehouses, etc. I've tried both, and you really need the latter scale.
Quick example on that last point: To the right you'll see a photo of the main gate and flanking towers of the Keep on the Borderlands, which I modeled at the indicated 1"=20 feet. Note that one figure of crossbowmen nicely fit atop each tower (per the text, there's a total of 8 such men with crossbows in each tower). If the scale were instead 1"=30 feet, then those towers would be smaller by a factor of 2/3, and the figures would no longer fit there.
So -- It looks like 20 feet per inch has a preponderance of evidence in its favor.
Why 30 seconds per turn?
Again, we can reason from Chainmail; if 1:20 action takes place at 1 minute intervals (again, pretty reasonable), then 1:10 action should have turns of about half that length, or 30 seconds. More-or-less.
Or, more importantly: It turns out that the "golden" number of rounds-per-turn for our game is 3; that is, we say that 3 rounds of D&D action is equal to one turn in Book of War. If we use a source like Holmes that says 1 round=10 seconds (the most reasonable that I've seen), then clearly 1 turn=30 seconds here.
One of the really nice things about this is that it's coincidentally (?) about the same as our increase in distance scale, above. If you play D&D at 1"=5 feet (and I do), then our BOW scale is 4 times that (1"=20 feet). And you can see here that our time scale is itself 3 times that of D&D. So I call these numbers "approximately equal", and since we've increased both time & distance by about the same factor, our Movement rates in inches stay the same. If a horse runs 24"/round in D&D, then it runs 24"/turn in Book of War. That simplifies things enormously.
Now, let's say you disagree with my interpretations of D&D scale (like 1 round = 10 sec, per Holmes), and you want to stick with the legacy of 1 round = 1 minute for man-to-man scale. Well, that's not a problem; the far more important issue is that 1 BOW turn = 3 rounds of D&D. So for you, 1 BOW turn is 3 minutes. Everything else stays the same.
Next time: Why the statistics magically work at the conversion rate of 1 BOW turn = 3 D&D rounds.