Spring , does you double regulate well, but is simply shooting 3 1/2" low? If this is the case if it is regulateing at 50 yds, try it and see how it shoots at 100 yds. Then see how it regulates at 10 yds, If it is on at 10 yds, and 100 yds, and low at 50 yds, it may be just about perfect, depending on caliber. If it is still regulateing at 100 yds, but is still low,there and at 10 yds, then you must lower the front sight. Be very carefull here, a little goes a long way with that file, and you don't want that rifle shooting 3" low at 10 yds!

I'm trying to remember what double, and chambering you have! The type of sights will make a difference in how you adjust the hight.

While I've got you here, do you have a wing shooting, or a shotgun book out?