Let me get this straight: you have a flat bar of 5mm thick metal with five holes drilled in it. You bend the bar into a circle (hoop), but when the ends meet, completing the circle, it's too big for your purposes, but the holes align with the holes
on the drum. And you want to know how much to cut from the end of the bar, so that it fits the drum better, and maintain the position of the 5 holes relative to the holes on the conga drum? Does that state your problem? If so, you are in deep doo-doo. First, we'd need to know by how much you want to decrease the diameter of the hoop....
a foot? 1/10th of an inch? But the basic formula is that circumference = diameter x pi. In your case, circumference is also the length of your steel bar. Cutting off 3.14 or so inches from the bar will reduce the diameter of your hoop by an inch. BUT your problem is with the holes. Their positions are fixed, but the distances between them will vary with the diameter of the hoop. Therefore, as you change the size of the hoop, you
gotta drill the holes in different locations. The
hole arrangement that works for a 12" hoop will not be the same as for a 10" hoop. Save yourself the headaches and just buy what you need retail.