OK- if you are worried about color bias, (I wouldn't be) you try one or both of the following over-kill options: First, get a piece of complimentary color gel for each filter (in roughly the same tone or opacity), measure the amount this holds back on it's own, add the filter and measure again, then subtract the amount of filter factor for the gel alone (or, alternately, average the two, esp. if you are confident of the opacity match). Second, you could test shooting film, and add 1/4 stop with each exposure in a bracket over a range which well covers the amount you suspect the filter/meter combo should be off, check the resulting film with a densitometer to see what exposure yeilds ISO conditions vs. non-filtered speed, and then record and use the difference in the resulting exposure as your correct filter factor.
Just recording these processes is exhausting enough to make me not want to bother. I have used and trusted the filter-over-the-dome testing method for years, and with some experience, will tweak the results depending upon conditions. If in doubt, add some exposure, but don't worry so much about it. This is black and white film, after all, and the exposure lattitude should be enough that you should get good printable negatives even if you are 1/8 to 1/2 stop off- and you really should be able to get well within this ballpark.
Oh- and the polarizer will hold back within 1/8 stop at all angles, so don't worry about what position it's in for testing; color-wise, it should be neutral, so you shouldn't have any problems with your color issues with your meter. Good luck.
