[Help] Trying to calc thrust effectiveness as a function of altitude from planet center

(New poster, go easy)

I'm a decent coder but relatively new to C# and math at this level is pretty shaky so any help would be appreciated.

Using this as a reference and the closest I come to a formula is something like: -0.0000622190421378462*x+0.84 where x is Altitude (distance from planet center) and gives a percentage of thrust effectiveness.
I used a line of best fit...

[Help] Trying to calc thrust effectiveness as a function of altitude from planet center