chart: how much extra att you need to be equal to 1 extra %stat

change in range per change in X = partial derivative of range equation

Set the partials equal to each other and solve for dX when dY = 1 to obtain the change in X that is equivalent to a 1 unit change in Y.

Close, but no cigar. This would tell you WHEN an increment by one in either stat will result in a similar change in damage range, not how much one of them should change by to match the unit change of the other. Key: Setting the partials (the amount of change of damage per UNIT change in each variable) equal to each other.

[spoiler=maths don't lie][/spoiler]

I believe what we have to do instead is to integrate both sides of the the partial wrt primaryStat from currentDmg to finalDmg to get the change in damage to correspond to the appropriate change in primaryStat.