You can't pinpoint it, but you can create some confidence intervals.
Just making up numbers to show an example, but maybe your FIDE rating has a 50% chance of being within 100 points of your chess.com rating-100, and a 75% chance of being within 200 points of that, etc.
TheMouse, yes of course. In my previous posts I have listed many reasons why ratings don't translate and this is another important one. The point remains, you won't be able to say rating X at chess.com equals rating Y for FIDE, USCF, etc.
Perhaps you could derive some kind of formula that incorporates statistics like average number of games in progress and time per move. But good luck quantifying how seriously players take different formats. Even if you could, you would wind up with such an unwieldy formula it would be useless.