What is the lowest voltage that I could get with an i5 at 4.2Ghz most of the time.
Depends on your cpu only way to find out is to try it and test for stability I would guess around 1.1-1.15v. I have my i7 3770k @ 4.3 at 1.155 v
I tested it in prime 95 for 8 hours at 1.050 before it erroerd once so what would my next logical voltage be
I would try like 1.08 if it lasted that long in prime. I use OCCT personally