While fatigue works on a linear model and it's easy to calculate the level of fatigue for your players given a certain # of PA or pitches, I'm fairly certain the actual effects of fatigue don't operate on a linear model. I've done a large bit of experimenting with fatigue for both hitters and pitchers (and detailed a large bit of it in the forums way back when) and find that the effects are relatively minimal going from 100 to 70 and from 30-0, but the difference between 70-30 is quite substantial. Not there isn't a performance drop between 100-70, there is, and a good drop at that, but a player at 70% will still put up fairly respectable numbers that, while lower than what they'd put up at 100%, are still very usable and depending on other factors can still even be competitive. The difference between a player at 70% and 30% is huge (I'll expand this more later), but again, the difference between 30 & 0% is again fairly close. There's a difference and the player at 30% will put up noticeably better stats than the player at 0%, but in the end, they're both just horrible and neither is going to help you win many (any) games.
The way I like to think of it - though these are just purely rounded example numbers - is that between 100 & 70% you have about a 15% drop in performance, from 70 to 30% you have about a 70% drop in performance, and then from 30 to 0% there is also a 15% drop in performance. This is most easily seen in pitchers as it is easier to control their fatigue levels than it is that of hitters (as it's easier to control how many pitches your pitcher throws than how many PA your hitters get in a game). Some of my early fatigue strategy tests and teams were built on the premise that these players would still perform at a competitive level at as low as 80%. And some of my games played tests for pitchers operated on the idea that a pitcher could go as low as 70% and still be relatively effective (And by that, I mean a ~1.00 WHIP turning into a ~1.20 WHIP).
As a purely anecdotal example, I drafted the 1918 Babe Ruth onto a $40m team (solely to prove a point to schwarze about the available player pool in low cap leagues in the hope of him expanding the WISC to include some lower cap leagues and to occasionally change the cap in the Exclusive Ownership league (when he ran it) to a lower cap) and was a borderline playoff team down the stretch run. Ruth was getting fatigued, but was easily my best hitter, so I slowly bumped his autorest down from 93 to 90 and then to 85. By the time we secured a playoff spot Ruth was down to the low 80s. He played throughout the whole playoffs while in the blue and down the stretch (about 20 games plus the playoffs) in which he spent the vast majority of the time blue he put up #s substantially better than he had all season. Now, I wouldn't expect that, and it was certainly an abnormality, but it still goes to show that a fatigued player can put up great numbers.