Only 12 weeks and 24 weeks have been tested so we are into the realms of speculation here.
If the chances are linear with time then your conclusion would be correct, but is that so? I don't know. How long past 12 weeks does it take to eliminate the last group of resistant mutations capable of rebounding back into full HCV infection, if they are indeed still present at 12 weeks? There are different kinds of variants with different fitness levels, so which kind are still present and needing to be knocked off? Some kinds might go in another 4 weeks, some kinds might take longer. How long is a piece of string? In the 24 week tests we don't know if the last virions standing were still standing right up until the day before the 24 weeks were up, we just know that they were no longer infectious after week 24.
Wish I could give you more of the certainty you are looking for but I can only add that this virus is a tricky m.fucker and the one thing there's not a lot of in its treatment is certainty. Not yet anyway. For this reason I am doing 24 weeks rather than the 12 recommended for my circumstances. Even then, I won't be sure until I get my SVR24, so you are talking to the completely wrong person for an assurance that less than 24 should be fine. That's a roll of the dice I'm afraid.
So you can do the 12 weeks and take the 95% and see what happens, then retreat if you are unlucky. That is a valid option. Depends on how that fits into your life. For me I'm 65 with 2 failed treatments behind me. I really need to finish it this time, whatever it takes.
My very best