Let’s assume for the sake of discussion that GILD’s pricing is linear with respect to duration.
Certainly your approach follows given that premise.
Of course, there is precedent for non-linear approaches. GILD could charge a set fee (cap it at 8 weeks) for the treatment term regardless of whether it is 8 or 12 weeks.
It seems that GILD is most likely to maximize revenue by avoiding a weighted discount at 30% - say holding it down to 10-15%.