Compute the secondary attribute modifier for a given index, where each step above the baseline of 5 adds +5 (and an index of 0 or below yields -25).
The attribute index (truncated to an integer).
The corresponding ±5-stepped modifier.
Compute the secondary attribute modifier for a given index, where each step above the baseline of 5 adds +5 (and an index of 0 or below yields -25).