-First ride bikes really fast for a long time- sublimating the physics and anatomic issues at hand is the #1 thing. It's a 3d world with 5 degrees of freedom and whole lot of bones and muscles. Understanding that shit is important, as is being able to picture how small changes make larger differences.
-Secondableeey converse as much as you can with a customer without having them give you numbers or pictures. Listen well and try to steer the direction to place where people might open up about percieved weakness and defficencies. That gives you a datapoint to work backwards from once you see how their bike is currently setup. Datapoint- as in one hardpoint in a complex system. So many damn linkages and muscles.
-Thirdably reconcile what they said with what you see. Cognitive dissonance may be one of the largest reasons people seek a made to measure bike. If it looks wrong, it is wrong. Draw up the bike they have now while you're at it. Get inside it. Would you do something differently? Is it worth doing something differently? Maybe the rider is just too short and too high on the bike and 80% of their physical complaints are a result of wonky vehicle dynamics. The large majority of people are already on the right bike that's setup wrong, a key thing to keep in mind.
- Stuff goes on from there but as far as I'm concerned there isn't much more you could do in person besides working on knee tracking issues, leg length discrepancies or tiny bits of cleat setup.
I haven't been selling these things long enough to know what level of service to offer at a given price point, or how much of that bullshit to build into the cost of the frame. Sometimes it takes 20 minutes to draw a bike, sometimes it takes me prying information out of people, making free-body diagrams, closing my eyes and spinning around or eating two burritos at once, all difficult, time consuming tasks.
It's safe to ignore everything if an experienced racer comes to you with a set of numbers and asks you to build it. Just make sure it's not wrong first and find out why, for some reason they want you to build a bike.
It's not your place to fuck with their numbers unless they need to be fucked with. If they've been winning crits since the early 90's they're doing it right.
Unless you've ridden and dined with the "pro" "fitter" that a customer is talking about, just take the sheet of paper and say thank you. You can look back at it later to see how things might be different. Lot's of these guys went to a "fit school". I went to Space Camp. Those jerks at NASA never called me back even though I aced the fake landing in the fake shuttle.