I usually figure a position guy can start about the same percentage of games, or a bit more (at 100%), as his Durability #. So a 64 guy could start (roughly) 2/3 of the games at 100%,
Roughly:
A guy with 70 Durability I sit 3 out of every 10 games. A bit less when the playoff run is on.
A guy with 80 Durability I sit 2 out of every 10 games. "" ""
You get the idea.
It works pretty well if you try to keep guys at 100%. I do.