[Jim S]

You'll have to ask that of the guys giving the clinic. We don't know where you are going, who is giving it, or what they intend to teach.
Having been an umpire for 29 years, it's the most fun place to be.
My recomendation to new umpires is that you start 8 or even 9 yards back at first. Get used t the players coming at you and what it looks like. If you were ever a linebacker you know this and can probably start closer.
But get used to what you are seeing and the flow of the game at the position before you move up closer. One the play is over go quickly (not batass out of hell) to the ball, if not too far beyond the hash. But don't go into the pile to quickly. Give the players a chance to move.
They are going to anyway and you don't want your head down there when the helmets start coming up. Besides, if you're in too close you won't see a lot of what goes on around the ball.
One last thing. When a running back comes at you you can back out, play matador to just stay still, the RB will avoid you. But watch out for QB's! For some reason QBs think they have to try to fake out the umpire as well as the defense. Then they get pissed when to let them have the place they faked to , and run into you in the other.