I think the leg falling asleep reflects the tightness of some muscles around it, including the lower back. This might pull the leg together in a certain way where the sciata nerve becomes compressed... it is a large cord of nerve that runs down the back of the leg, a picture .
I'm able to sit in half-lotus without the leg going numb... but I've found that this doesn't seem to carry over into the full lotus, where my leg (or both legs even) will go numb. I can sit in full-lotus quite comfortably... but still the leg goes numb after 15 minutes.
I've tried to apply my experience from the half-lotus to full-lotus, to try figure out how to keep the legs from going numb, but that doesn't seem to work. So, I experimented by stretching in several ways, just by trial and error, and when I lied on my back, while keeping the legs in full-lotus, I found some muscles that I never knew that I had. They were quite tight, and deep in the back of my thigh... I think that this is probably the cause.
Just experiment, learn how to relax the muscles in the legs. Don't allow any unnecessary tension to develop. Try to figure out how to position the legs in a way where the nerve will not be compressed. I actually learned all of that just by trial and error, with some reading.
Lately, there haven't been enough free time for me to go into the physics of my own sitting as deeply as I'd like... so it has been just regular samatha meditation for me. Good luck with your practice.