I know the title in confusing. I'm very sorry. I have no clue how to word this! Its for a small project that people need me to do. So, let me cut this up into smaller bite-sized bits.

I am trying to make 15 be a base number. what does that mean? Well, Im working on a small thing where you use a d100 to hit something, right? and 15 is the natural parry range for a weapon. you can spend points to upgrade or downgrade the range.

so, if you move your parry range to 13, you would have a cost of 1 point, as it goes up in cost ever even number. like so:

17 = -1. 16 = -1. 15 = 0. 14 = 1. 13 = 1. 12 = 2. 11 = 2.

Now, I need a calculation that observes a cell, and if its 15, it says 0, if it says 16, -1, if it says 3, 1. Pretty easy to understand. Now, I could use IFS, but that would be ALOT of ifs statements, since the parry range goes from 1 to 50. any ideas?

Edit: Link to throwaway sheet