Not sure about gun fitting but try it on a pattern plate as long as the gun is shooting where you are looking the rest is down to you put a cross or mark on the pattern plate stand back about 15 paces dont try lineing up the bead look at the target and shot it.
Remember 2/3rd of your shot is always ubove your taget.
Once you have done this you know your gun fits so stop thinking about it.
look at the target not the bead remember some of the top shots dont even have a bead.
you didnt need it on the pattern plate!
Practice you gun mount every night in the mirror use your eye as the target and you should be centure of the rib every time your muscles in your body will remember a good mount.
if you mount the gun different every time then you need to practice this before you go near a pattern plate make sure your eye is in the middle of the rib
Hope this helps let me know