http://iscoresports.com/

Batter hits a solid line drive to right field. Right fielder fields the ball and throws it to first. First baseman makes the catch in time to get an out but his foot is not on the base. Hit or error?

That's a 'had to be there' call. The standard is if the play requires 'ordinary effort'. If 1B was really having to work hard just to make the catch, then it probably is not an error. If 1B just had to stand there and make a catch in plenty of time, then it probably is not an error. The final instruction in the rules for scoring is that you are to give the benefit of the doubt to the batter.

You should also take into considertaion the age of the players and try to apply your standard consistently. (i.e. if it would be an easy play for your top player, but most kids would require extra effort, I would score it as a hit.) Also, it the timing was close, sounds like it's a legitimate hit. At the end of the day, scorekeeper's judgement.