The old ECF to ELO formula was (ECF * 8) + 600 = ELO. Using this formula an ELO rating of 2000 would be equivalent to an ECF grade of 175.
However, a new formula is now used for ECF grades below 216. This is (ECF * 5) + 1250 = ELO. Using the new formula an ELO rating of 2000 is equivalent to an ECF grade of 150.
I've always understood that USCF ratings are reckoned to be 100 points higher than ELO ratings. However when I checked the current top US 100 players: http://tinyurl.com/ymq3dn it seemed to me that they might only be about 50 points higher (e.g. Kamsky has a USCF rating of 2753 and an ELO rating of 2705).