Being 95% sure will be your call to make, it's kind of a subjective thing in cases where randomness is involved. After all, doing 100% HQ on tier 1 for 50 straight synths is not technically impossible, it's just very very very unlikely. Just keep doing your synths and track how much HQ you do. It will eventually trend towards a certain percentage, and I do believe it is accepted to be 10%. I haven't done extensive tests on tier 1, but from what I have done, 10% seems about right. But you won't be able to measure how sure you are in percentages, I don't think.