After finding the right cross section equation, (whatever it is,) revolving it seems appropriate.
No, I disagree with myself. Variable Reflectivity may be expressed as an equation???

- Brian

https://physics.stackexchange.com/questions/309378/what-is-a-gaussian-mirror
"A Gaussian mirror is a mirror whose surface profile is in the shape of a gaussian function..."
"To first order, a gaussian is a parabola (exp(±r²) ≈ 1 ± r²) and ..."