1-Umpire mechanics question

[jTheUmp]

Situation:
Bases Loaded, 1 out. I'm PU, working alone.

Batter hits a sharp line drive to the right of F6. F6 takes a quick step to his right, and gets a glove on it enough to knock the ball down. I take my step out towards 3B in foul territory. F6 recovers the ball, throws to F4 for the force at second. F4 throws to F5, while I move about 5 steps up towards 3B to cover the tag play. F5 tags out R2. Inning over.

Head coach, on his way back to the dugout, asks me if F3's run counts or not.

Gulp.

Obviously, if R3 had touched home plate prior to the tag on R2, the run counts, and if R3 hadn't crossed home place prior to the tag, then the run would not count.

The problem is, because of my positioning at the time of the tag (about halfway between 3rd and home, about 2 steps into foul territory, looking directly at 3B), I had no idea about R3's status at the time of the tag.

So, my question to all of you... where should I have been to get the best look at all of this action? (And unfortunately, "hire a BU" isn't an acceptable answer, as I don't get to make that decision).