I'd start with say 1.3v Run Aida64 for a good 30mins if stable you can try lower til it stops being stable. (30mins does not guarantee 100% stability, but it's good enough for initial testing purposes to find your voltage range).
If 1.3 is good, I'd try say 1.25v to see how you go on the low end. If it's not stable here, you know your range is somewhere between 1.25 or 1.3 for stock speeds. For reference my i5 4690k at stock (3.5/3.9 stock/turbo) was happy to run at 1.05v. at 4.2ghz i'm at 1.16v but i'm running an i5 of a generation prior. I imagine 1.25-1.3v SHOULD be achievable for stock clocks on the 6700k.