Im not really sure if I understand what you are talking about but here is my explination if I do.

Lets say you normally set that target up at 100yds. If you want to set it up at 50yds then you would want to make it half as big to simulate the yardage difference since it is only half as far away. For 25yds you would make it half the size of the 50yd one for 75yds you would make it 150% as big as the 50yd one which would really be 75% as big as a normal one.

If you would like to simulate longer ranges then you still have to shrink it. Again say it is normally shot at 100yds and you would like to simulate 500yds at a distance of 50yds. First you would make it half the size because you are only shooting at 50yds, then you would have to shrink it 50% for every extra 100yds you wanted to add to distance. So for 200yds simulated at 50yds it would be shrank to half then to half again. For 300 you would have to shrink it to half again and so on. If you wanted to simulate 500yds you would have shrink it down 5 times each time making it half as big to make it look correct.

I hope that is what you were asking for.

Michael

Here's what I come up with:

@25 yds. = 1.05 inches tall X .85 inches wide
(25/500 = .05)(.05 X 21 = 1.05)(.05 X 17 = .85)

@50 yds. = 2.1 inches tall X 1.7 inches wide
(50/500 = .1)(.1 X 21 = 2.1)(.1 X 17 = 1.7)

At least I think that's right........

You may want to also figure in where the bullet's supposed to strike at 500m and mark that on the target.  That way they have to use the correct aiming point for 500m instead of 25m.  Of course with the Elcan I guess it wouldn't matter, just dial in 500m and aim dead-on.

Ross