the math part is really just to find the MAC... which is going to be the average chord length. The average chord then is as you suspect: (Cr + Ct)/2
Cr = root chord
Ct = Tip chord
Then its a matter of finding where you want your cg... a normal good starting point is 25%, so once you find your MAC, you use 25% of that as your cg... For a constant taper, your mac usually falls dead center on the wing profile.
The way we do it in my Aerospace engineering classes is this:
Draw the planform for the wing (top view outline). Find your MAC (mathematically if you wish), and if you like, you can verify it by measuring the chord at the center of the wing... From there, mark a dot on the root chord at 25% chord length (1/4 of the chord length from the leading edge. Then do the same at the tip. Connect these two dots to find your quarter-chord line (this is a very important line). Now, draw a line perpendicular to the centerline of your plane from the intersection of your quarter-chord, and your MAC line, to the centerline of your plane... follow? This will give you the exact location of where you want your cg to be on your model... You can do it mathematically, but you'll have to do more or less math depending on the shape of the wing... and to do it graphically works really well, never screwed me up so far