I hate to sound like stickler, but you should get a "no-go" headspace gauge... much cheaper than fixing an OOB explosion, or any hospital visit. Just isn't worth it IMO to guess.
That said... ahem... you should be sure that the rim area of the cartridge is the only exposed area - this is where the thin sides of the case (blowout area) and the bottom of the cartridge (thicker area) begins. The idea for headspacing on the BACK of the cartridge is to ensure the thin-walled sides are almost completely covered, thus eliminating ruptured case problems.
AK headspace is ~ .005" - that's a feeler gauge measurement. That's where the masking tape "trick" comes from. Personally I'd use a feeler gauge first. The fact that the 7.62x25 is a lighter cartridge doesn't eliminate the need to headspace properly. If you check a reamer company's website like Clymer, you can get the measurements for the chamber. Not sure if that would help.
I'm still of the opinion that a headspace gauge is the best bet. You get it down to where a "no-go" gauge cannot close, and just loose enough that the action can close with an "average" factory round.
Not sure if this helps. I don't think you're way off but like I said, I think gauges are cheaper than fixing broken guns... or faces...