Concept of cumulative dosage

I’ve been in Accutane for almost exactly 12 months but my dermatologist says I still have a good 6 months to go and I’m so over it.

My dosage has been the following: Month 1 - 30mg Month 2 - 40mg Month3&4 - 60mg Months 5 - 40mg Months 6 thru 10 - 30mg Month 11 - 40mg Month 12 - 30mg

Why has my dosage fluctuated so much you may ask, I’m super active, running, ultimate frisbee, moderate weight training and even at 30mg my joints and muscles are incredibly stiff, when I go above 30mg my shoulder hurts, my wrist hurts, and I get this weird pain in my upper spine when I sleep.

I used and online cumulative dosage calculator which indicates I’ve reached my cumulative dosage but my derm says I 6 months if I stay at 30mg. Haven’t broken out in 4 months at this point.