This might be really basic. I'm trying to look for tension formula for tension of a linear line curve for envelopes in jsfx.

The common linear curve is distance/range, and I used (distance/range)^power to add tension.

However, the linear curve in REAPER looks a bit different:

Reaper Linear:

Max tension:

Min tension:

While with the linear ^ power:

Max tension:

Min tension (which looks weird):

Thus my question is, what would be the formula to create a curve with tension which is used for envelopes in REAPER?