Plays at 1st Base

[VanStanza]

Hi Guys:

As an umpire of 15 years, I feel I am a strong official interms of mechanics, judgement, etc., with a very good plate game. I do not feel my base game is as good, and so I'm looking for some advice regarding taking plays at 1st base.

Assume no runners and a ground ball to the infield. As the BU sets up at an ideal 90-degree angle to the throw, and the throw is coming across the infield, tell me as exactly as you can, what you do next...

I ask this VERY basic question because I think I am victim to calling runners out at 1st (who may be safe by a quarter-step).

I need help!