up to 3 point overcharge i use random % gems and as long as all four are not 94% it has so far been ok. have done 3 point with 3 94% and a 96%.. only time i lost one was when i did a 3 point with 4x 94%gems.
Generally though in a 1 time make of four gems u will get at least 1 highish% gem in the mix. if not just make another
4 point overcharge i'd say all 98+ and 5 point overcharge 3x99% at least .
have done many in my time and only lost items when i have been pushing the limits
so far i ran good with average gem quality like 99% for 5pt OC, 98% for 4pt OC 97% for 3 pt OC...
i always went this formula but had some blowups on bg-stuff with using 94% gems. so i decided to use no more 94% for OC>3, as if u are familiar with Leladia's calc, you will have mentioned no difference in OC chance by using 94% or even 95% gems. In fact i think there seems to be, and so i just stripped 94% qual for OC.
Don't take the 30% chance serious, or let you lead by that !!!
In my younger SC career (when i was 800+ SC) i did already quite some 5pt OC's. if you know leladia - again - you will know that a 5pt OC with a master SC gives 10% chance of success, in fact i made around 50 items (bg stuff with 5pt OC + also 2-3 sets of lvl51 stuff) and i never had any boom