SpudFiles

I think it's an area calculation, in which they set barrel cross-section and the area af the "wall" of a cylinder that would fit in the gap (if you will imagain that). Assuming this is the case...

pi*R^2 = pi*2R*G
Where R = radius and G = gap

deviding by pi and one factor of R gives "G" on one side gives
R = 2G
1/2R = G

Or, in terms of diameter
1/4*2R = <b>1/4D = G</b>

But, now, we are stuck wondering "is this truely sufficent?". After all, compressable fluid flows are complex beasties, and surely can't be modled perfectly by a simple area calculation.
For this reason, I'd err a bit on the edge of caution, and use a bit more gap than the calculation gives you. It doesn't affect preformance as much as some would try to imply, or at least the GGDT doesn't think so...

Maybe so but TO much movement could affect it, or so I think. If the piston moved back a lot like say a few inches then the air woul fill that space as well as the barrel leaving less pressure to push the ammo.

You might loose a couple psi from slightly overzelous piston movement - but, that extra chamber volume thus created will cause the preformance to be a bit higher than just operating the cannon at 97 instead of 100 psi.

With such small gains from sticking exactly to the formula (due to pressure loss), there are only two major concerns in letting the piston move farther:
1) will it increase opening time noticably?
2) will the increased piston velocity break the launcher?
...and a couple very good reasons to do so:
a) it ensures that it will move back 1/4th of the diameter
b) it increases the tolerance for error (basicaly "a" rephrased)
c) if the formula isn't perfect, it causes fuller flow

1) will vary gun-to-gun and with pressure to some extent (ggdt!); 2) is a legitimate concern... which is why I said a "bit more" than what the 1/4 formula calls for instead of "10 times more".