90º is great on a force play, but might not be the best for a tag. For example, think of a bunt fielded by the catcher and thrown to first. If you are 90º, approximately in the line between 1B and 2B, and the throw was off, I would think there's a good chance of not seeing a tag. I've been taught in the above situation, that when I see the throw will be off, to move toward the foul line for a better angle.
I personally think the extended path of the runner, e.g. third base line extended for a play at the plate, is a great place to be... adjusting as I see the angle of the runner. In 2 umpire mechanics, this could lead to a problem of me being out of position to get down to third rapidly if I have a play there.
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Dan
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