if someone says they have a 20" kick drum and I send them a vinyl sticker that is 20" in diameter is that going to be enough to cover it entirely or should it be 21 or 22"?
The max I would go as a general rule is 1" under for lettered graphics.
A 20" head for example will yield 19.5" useable, but that would be cramped for some graphics. I just measured an 18 and a 20, the max I would make it is 1" under and even that might be too cramped depending on the graphic. Im sure you have a computer program which allows for a circle and the graphic inserted, see what looks cramped and what doesn't.
Now if you're talking about a skin which covers the existing head entirely, then go with the .5" under, 20= 19.5 18=17.5 etc. At the -.5" formulae there is a 1/16" to be played with, vinyl being what it is, that 16th could matter.
My experience with vinyl is that it would be difficult to accurately place a 'skin' type graphic over the entire head surface. At the thicknesses you'd be talking to not seriously affect head tone it would be difficult for the average Joe drummer, the stuff stretches.
The market IMO is in need of fugitive graphics, a band name/logo that could easily be slapped on and taken off. If one is going to graphic a BD head as in 'permanent', they'll usually send it off to get done professionally/right. The do it yourself stuff should be fugitive, with benefits the benefits being multiple uses and an allowance for error in the application.