I have played against such players from Russia. The Russian rating system is not too dissimilar from the FIDE rating system. He very likely is at least 1900 in USCF rating strength. He should be adequately prepared in his openings and will be skilled in endgame technique.
I’d follow the rulebook and add 250 points. Even if this puts him too high it will give the player valuable experience, and he’ll get a USCF rating which he can use in future tournaments.
I did this at the Massachusetts Open with a player who told me he had a Philippines rating of 1980. I followed the rulebook and added 250 points, putting him in the Open section. His post-tournament USCF rating is… (wait for it)… 1980! In this case it appears that adding 0 would have been a more accurate formula for converting his Philippines rating to a USCF rating than adding 250. I still think I did the right thing. If I hadn’t known about the Philippines rating he would probably have gone 6-0 in the U1500 section as an unrated.