I would agree with you that it seems to be linear, but data is very limited all guys that really matter are between repscatable and prominent in form all the time.
It looks to me that guys that constantly ship in between 48-90 minutes have an expectansy value of strong, and that the variation is low. My 48-90 minutes have almost always been evenly distributed between resp/strong/prof in GS with the exception from when I tested to train Game Shape, then the distribution was evenly between strong and prof if I remeber it correctly.