How many games to get a good idea of computer chess board rating? I have an old Radio Shack 1850 Electronic Chess Computer, which I would like to know the ELO rating each level plays at. The instructions just give average number of seconds per move.

I figure I would play it against tChess Pro on my iPad, which the engine can be set to a specific ELO rating. When I find they are both winning about the same number of games I can check what level tChess is at and that will be about what the 1850's rating is.

Question is, how many games would be a good sample before I make that call? Obviously if one dominates the other, I'll have to tweak the levels and retry.
There is no perfect answer here. First, and it is for a reason, FIDE says that you need 9 games to have an established rating, but the USCF says you need 25. Obviously, the more games, the closer the rating will be to accurate. That said, to really get a more accurate rating, you also need to vary the competition, and have it play against various rated humans.

I think that it is probably too weak to pair up against any modern computer program that is rated. It would get slaughtered, and not help you determine how strong it really is.

I would say at least 10.