I'm making a character sheet for an RPG. I have an attribute value in B, a modifier in C, and a save in H. I need 2 functions.

First function: I need a function that can look at the B value and calculate the correct value for C. The range is 3=-3, 4-5=-2, 6-8=-1, 9-12=0, 13-15=1, 16-17=2, 18=3

2nd function: I need to find the value of H by using the max of 2 values calculated for C and subtract them from "15". H are taller boxes than the rest(H2:H3 1 box, 1 value)

An attempted visualisation. B,C,H are the columns for the functions, 2-7 and the row numbers.

........B...........|...C..........|..H...

1..|Attribute|Modifier|Save
2..|STR.....|X|...............|.1st H
3..|CON...|X|...............|.value
4..|INT.....|X|...............|.2nd H
5..|DEX ...|X|...............|.value
6..|WIS....|X|...............|.3rd H
7..|CHA...|X|...............|.value

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Thank you for any help and I hope my visualisation is able to be understood