Help needed

The idea I want to do based on this logarithm

Basically the idea is, have a linear progression of reps (+1 power per 100 reps you have), until that line intersects (around 640~), then it would follow the curve, which would reduce the progression of rep power.

My problem is translating that into php, there’s a natural logarithm function built in still at a loss how to apply the curve.