WARNING: Alpha stage, not supported by anyone else than me. I have not tried it seriously, just with e. g. +3 and -4 glasses in top of -7 contact lenses.
Why drop from SE
SE correction is commonly used for mild astigmatism. There is a fear this can cause axial elongation, but I now discard this fear while I also was between fearers. The goal to drop 0.25 (when mild or moderate myopia) or 0.5 (when high myopia) diopters from your full correction SE is to get a better visual acuity and leave an opportunity for Active Focus (I will tell you how this could happen in me and you).
Cylindrical lenses correction is sometimes unreliable when irregular or asymmetric corneal astigmatism comes into game. Like in mine, I get 20/13 with Cyl yet if I reduce Sph I hit by shadows which even can have orange yellow or light blue colour.
Why not just without cylinder
It can work very well with ≥1 D Cyl, but VA can become too low when Cyl is ≥2. My case also involves eye strain when there is no hyperopic defocus visible.
You see both Myopic and Hyperopic defocus, double vision or blur is mostly vertical (if WTR case). Then, you need to reduce Myopic defocus by aggravating Hyperopic defocus. It’s a some form of constant drill: you need to train reverse accommodation. My right eye reduced about 0.5 diopters of SE (-9 to -8.5) after doing that for a couple of minutes.
When to reduce
If you see blur is equal on all meridians (no double vision, just equal + and - on less and more myopic meridian, respectively) then you need to drop another 0.25 or 0.5 Sph.
It’s from Ortho C lens fitting: it is made to be at least 0.5 D flatter than corneal SE.
Full correction contact lenses: R -7.50/-2.00x180 L -8.00/-1.25x180
SE: R -8.50 L -8.50
SE reduced to: R -8.00 L -8.00