I have a simple and foolproof system:
1. for relic mats, the 1st two, do as many as possible at once. the harder to get ones do in 3 or 4's, just not all at once.
2. for character shards on normal energy or cantina 3 works best
3. for gear on normal it's usually 2, sometimes for lower ranked gear you need many of do 3's.
4. for hard/ship nodes do 3, then 2.
5. it's all math and the number of attempts doesn't matter at all, but as Jarvind noted this makes me feel better and seems like it works better and I avoid the crushing feeling when simming a bunch and not getting the "correct" rng.