TBH counting has been a great difficulty for me whether it is 0 or even 1.
c) 2 ≤ i ≤ 12
would pretty much force me to use fingers.
That’s actually the easiest: (upper - lower) + 1. In your example: (12 - 2) + 1 = 11.
Yup. Off by one errors have a way of screwing your head I guess :)