did i handle this correctly?

[joereed]

Scrimmage kick, R fields it at the 40 going out and procedes to return it for a a touchdown. It is at K'2 2 yardline and appears to be a no brainer touch down. Meanwhile R78 procedes and pushes one of K's players in the back all the way back at the 50. I ruled it as a personal foul and counted the TD and tacked on the 15 at the kickoff. Did I screw up by throwing the flag as soon as I seen it the foul (thus the returner not in the endzone yet) Or should I have waited until the returner was clearly in the endzone then throw the flag? I'm assuming I made the right enforcement logically , however in correct terms , the enforcment was wrong because the player had yet to cross the goalline? The only thing I can think I did wrong was not wait untill the obvious td was scored before making this call. 2nd year official