Quote Originally Posted by Robert Martellaro View Post
“In response to poor fitting in the form of tilt or decentration, lenses with aspheric form surfaces were found to have greater off-axis power errors than best-form lenses with spherical surfaces.” -Atchison, D. A. & Tame

https://www.ncbi.nlm.nih.gov/pubmed/8278198

https://www.opticianonline.net/news/...lar-refraction

https://www.odob.health.nz/wp-conten...f-D-Wilson.pdf
Nailed it, best form base curves can be estimated with vogels rule, thats where plus 6 comes from it's the guesstimated optimal back curve. demo lenses have a curvature to which you can clock, it comes out to being close enough to what the actual back curve is, then as Brent so eloquently put it the power error is negligible from there.