Suggestions:
A). Buy
Zoom Ellipsoidals. Lots.
B). Buy
Iris Kits. Lots. also Very Expensive.
C). Buy Aperture
Gobo's. However, no mathematical equation will land you with just the perfect-sized circle out of a factory made
gobo unless you are already using a
Zoom. I don't think the selection of aperture sizes is very extensive either. Having them custom made by
Rosco,
Apollo,
Gam,
etc... is absolutely crazy. Also, the trial and error process will eat your budget.
My suggestion: Go to Home Depot. Go to the roofing section and
pick up some 16 or 20 guage roof flashing or similar tin material. You can also get tin by the rolls, but make sure it's not aluminum. It won't last as long. Stainless is great, as is galvanized (I would imagine). Then measure the standard
gobo size used by your
instrument. Cut those squares (its easier then trying to cut them into circles like some real gobos). Now is where you need to get out the drill and some assorted sized bits. (you may even need a holesaw
bit. Make sure it's for metal though.) Then just experiment with different sized holes to see what you need for the right sized
template. Once you find the right one, use it on all your home-made gobos. Note that the heavier (smaller number) gauged metal you get, the longer the
gobo will last. I'm sure it will burn through if it's a long term solution regardless of what you use though. That should be a quick and easy solution to your lighting quandary. Hope it helps!