TL;DR - For the best actual red cheeks effect, you should be focusing on strapped, not spanked. The latter enhances things, but influences pose more than a spanked ass.
Rosy red cheeks brighten everyone's day. Well, compared to busted, bruised cheeks that only brighten some of our days.

You can start at 1.0 but don't be shy about ramping up the weight I still get coherent images ramping up well above 1.4. For animated models, you will need to set the weights high.

If you want images without a traditional 'spanking' posture use 'strapped' or 'strapped ass', which should affect the position / framing of the image less. Generally works better to increase weights on 'strapped' rather than spanked.

Starter prompt
<subject>, spanked, strapped