Always try to get the "more bang for your buck". So since each kyro comes around at 28 crystals for 1 salvage. If you get 2 kyros on 1 50 crystals refresh you in a sense saved 8 crystals. If you got 3 kyros from that 1 refresh you saved 34 crystals.
3×28= 84
84-50=34
I personally when having to farm kyros only do 50 crystal refresh when going after them. If you do 100 cyrstal refreshes you have to get 4 in that run in order to not waste crystals. With doing 50 crystal refreshes, you only need to get 2 per run to not waste crystals, anymore you get its just crystals saved. RNG can still be bad though and you'll get runs were 0 kyros pop. Then you'll get some were 4 or 5 kyros can pop on a 50 crystals refresh