I just got my Apex kit yesterday Steve, and was somewhat wondering the same thing. I printed up several pics off the net, to use as a guide. I don't have them in front of me at the moment, but it basically looks like midway between the gas block and front sight block, as you stated. If I can't get an exact measurement, I plan to literally split the difference, and put it exactly midway.
Have you come across anything different for an RPK build, as compared to standard AKM's, that someone should watch out for? I don't see anything that stands out, but as I said, I just got my kit yesterday. I've built well over 30 kits, so I'm not exactly new to this, but just figured I'd ask.