For Illustrious:

Updated version, should be better than previous one.

Use normal strength, from 0.8 to 1 for base images

If you want to img2img or/and inpaint, use 0.5-0.7 strength.

For PONY:

Arueshalae PDXL lora (trained on site)

Don't put too much strength - even 0.5 gives enough.