Mechanics Question

[varefump]

This play happened last night in a Fed. game.

Two outs, bottom of the 7th, home team down 4-2, runners on 2nd & 3rd.

Umpire was in the 'C' position over SS left shoulder. Ball hit to SS (not hard, but also not softly). SS fields the ball cleanly, glances home briefly, then fires to 1st to retire the BR. Throw is low (dirt level) and F3 kneels to scoop the ball. BU is straight-lined and cannot see F3's foot or the bag. Ball and runner arrive almost simultaneously. F3 makes a clean catch. BU cannot see the exact position of the runner's foot at the time of the catch because F3 has her body blocking the view. BU is 'pretty sure' that F3's foot was on the bag due to her not having to stretch for the ball.

BU called an out and the game was 'over'. OC comes and asks if BU can get help from PU. They get together and PU said he was watching R2 rounding 3rd and did not see the play at 1st. So play stands as called.

Is there an accepted mechanic that can be used in this situation to get the play right 100% of the time?

I advocate the 'D' starting position in this situation and therefore the BU would have been about 20 feet from 1st and in a good position to see everything. Our training instructor wants to use the modified 'C' position (halfway between ss and 2nd base).

This is a tough play and what can we do to make it easier (besides getting a third pair of eyes)?