After the confusing personality thing, it's time for an easy one.

Each turn on increases chemistry by +17.5. Since there is a potential for four turn ons (two on each sim) that gives up to +70 from turn ons.

Each turn off reduces chemistry by -22.5. If both sims are turned off, then that's a total of -45.

Skill-based TOs trigger at 5+. For example "Good Cook" kicks in at Cooking 5 or more.

If turn ons and turn offs occasionally don't follow this pattern that's because some of them are broken. I use tunaisafish's Attraction Traits Fix which fixes part of the problem, though some bugs are buried too deeply in the code to be moddable.