I think your pace attrition is too high.
For the 100, 30 seconds seems more right. At least.
I can only hit my 500 pace in practice with a lot more rest.
I'm with you on that set. I couldn't make many more than five 100's at 500 race pace on 20 seconds rest. Seven, maybe eight. And I'd be wrecked for the rest of the workout.
I actually use 100's on 20 seconds rest to pace-train for the 1000y free. (I can make +/- 20 reps). 16-20 x 75's on ~20 seconds rest correlates much more closely to my 500y race pace. FWIW, my pace difference between the two is ~5%.
"They're made of latex, not nitroglycerin"