Adjusting center shot on mag riser

[bswear]

I just picked up a mag riser (A)and really don't know how to adjust the sight window for center shot.    I think there must be an allen screw in there to adjust in or out but how do I know how far is correct for my shafts.  Any way to measure or just shoot and check flight? Thanks in advance!

Brent

[Ceb]

There is a allen screw you access through the hole on the side of the riser opposite the rest. I just adjust for best flight either visually or shoot through paper.