Hi vetrofragile,
Welcome.
Niacin is bad for your liver when taken in large doses and for extended periods. Some people can build up a resistance and are able to take 1000 or 2000mg per day. This is not our goal.
The goal is to take it ONLY before orgasm and a minimum quantity, just enough to produce the flush. The flush is more of a guide than a medical necessity. I have taken a quantity that didn't produce flush, but "almost", and it still worked. But it is better to take enough to produce the flush "for sure".
You are doing well to start out slowly and "find" your level. if 90 almost gave you a flush, 100 or 120 should be good. But I find that it varies. Sometimes with 3 hrs of fasting 100 gives me a strong flush and other times nothing. But sometimes 100 will give me a very strong flush, so I don't like to take more than the 100.
Sometimes I'll take 100 and if it doesn't give me a flush within 30 minutes, I'll take another 100. Niacin is in your system and "protects you" for several hours, that is you can have the orgasm from any time after the peak of the flush up until about 3 hrs later.
I'm not sure I heard from anyone who has had a flush on half full stomach, and am not sure how much you'd have to take.
Some have had luck with niacinamide, which is no flush, but I'm not so sure the the other no flush type works for POIS, it's too different chemically.
The reason that niacin isn't in an official category is that the mechanism still isn't clear. The "official" categories describe a mechanism. I think an idea is forming as to the mechanism, but it isn't clear enough yet!