I believe the term you are looking for is ELO rating. (a rating based on OTB tournaments calculated using a forumla developed by Dr. Elo).
This has been addressed several times and answers vary quite a bit.
Some people are better at correspondance chess than OTB and vice versa. It really depends on what kind of player you are, how much expirience you have OTB, how quickly you get bored, etc.
You will find a pretty even number that are higher, lower, or about the same when comparing ELO ratings to correspondence ratings.
Since we already opened the classroom, let's all have a seat and learn something. People often think that "ELO" is some kind of serect acronym. It's not. Dr. Elo was simply a mathmatician and active USCF player. He developed the mathmatical formula that is the basis for most ratings computation systems in place today. The name of the formula is written in all caps to simply differenciate between the player and the formula.