Just put it into an online damage calculator. I use this one.Ok, I'm not sure I'm doing the calcs right so...
How much damage will a standard Rotom-H do to a standard Foretress with Overheat?
Then, if it survives (sash or whatever) how much damage will that Foretress do to that Rotom with Payback?
http://pokemon.marriland.com/damage.php
I think the damage it gives is about 2 points higher than what it should be (because it doesn't constantly round down) but that shouldn't matter much unless you want to calculate the chance of OHKO's, 2HKO's, etc.
If you lose to someone with a much higher rating, you don't lose much and if you win against someone with a much lower rating you don't gain much.Depends if you fight on the ladder or not.
Sometimes when you lose against a player with a high rating, you won't lose much (I think)