Hi, you have to be very careful even doing that. Some people go straight from 0-100 at the slightest critique and see it as a personal attack.
In the context of meeting an SP or with sex in general, I find what works best is to praise when things are good. If somethings not done to your liking, say "I can't wait for you to do [action]" or I really loved when you were doing [action]", please do it again." I make it sound like i'm begging for it in a playful way. I always voice it in a sense that consent is required, not as a command.
If something really feels bad, i'll start with "I'm sorry" and carry on with something like, "this isn't feeling good for me" or "this kinda hurts. If it's ok, we can try something else"