Thank you for picking this up, please keep it up!
Yeah the boob "equation" isn't an equation, since there are values of x that equal to 2 different values of y, because of the nipples.
Normally I'd be against a relationship like that, but Koto is more mature than her age implies and their situation is an exception, a very extreme situation. I say go fight for your happiness MC, just try to avoid having sex before 18. Also don't abandon your daughter ofc, keep providing for...