So you cant compare postal to chess.com to otb to ICCF to fide to bcf or whatever. There are too many variables including but not limited to level of activity, effort and is that person currently active.
Why don't you mention the biggest variable here, which is computer assistance? Only Rybka or Fritz can turn an 1800 ELO player into a GM.
I have a friend that I've played 6 times OTB...and I have won all the games...but his rating on CC is like 1750...which is way higher than mine. I am not sure how all the different factors come together to make the difference.