Hi Nick,Here is my thinking on that, and I'd appreciate any commentary or advice.
First, I bought some large lots of Wratten Filters on eBay from someone who obviously has no more use with them. There is a large collection of some used, and some unopened, in varying strengths and colors.
I'm thinking of gelling my strobes with varying levels of Green and Magenta and taking shots of a large high quality grey card.
Just 2 things for consideration.
The Wratten filters are intended to be used in the optical path. I'm not sure if they will survive the strobe's light intensity, and they may significantly change characteristics with such use. You may need to consider some dedicated light modifier gels by Lee (some of which are available as High Temperature versions) if you want to modify your flash. Of course modifying the flash will also reduce it's output power.
The other thing is that many Grey cards are not necessarily spectrally neutral. Afterall, they are usually intended to determine exposure, not necessarily white-balance. So use a good quality grey card that has a known spectrally neutral reflection. I once measured the WhiBal cards, and they seemed to be reasonably uniform, good enough for white balance tests on digital cameras (which have limited UV sensitivity and usually a built-in IR rejection/absorption filter).