When should I of hit the foul?

[ATLBrvs326]

I was doing a varsity boys scrimmagae today and had a play that was in my opinion a little difficult. I am in lead ball side and the ball comes to A1 to the right of the block, in perfect position to shoot a layup. A1 gets the ball from the point guard at the top of the three point arc and he is faced in that direction. B1 is behind A1 with both hands on him obviously limiting his movement to the basket (A1 is trying to turn and shoot). I intend to watch the entire play, and hopefully A1 will get around and get the shot up cleanly. This continues for about 3-5 seconds and I am just waiting for him to turn around and play through the contact. Then B1 lets go and A1 falls back and takes 3 steps without dribbling and falls to the ground. I call foul because it was the defenses fault. The B coach goes crazy. The question is not whether I got the call right, I know I did. I just dont know if I got it at the right time. Do you think I should of got it right away, but if that is the case then wouldnt I be taking the shot away from the offense? What do you think. Also, when the coach asked what he did wrong, I told him he had both hands on the offense. He says that is a terrible call. I ignore, but should I have told him firmly, "We are not going to have any of that tonight" or just taken it with stride. Thanks.