I found that the built-in meters in my Nikons see color-filtered light differently than film.
For this reason, I meter the scene without a filter and use the filter makers specification for the number of stops held back by the filter when setting exposure.
Reckoning the adjustment in stops is more logical than using filter time factors. For example, if the factor was 2.5, there is no shutter speed that we can use to get that correction.
A filter with a time factor of 2.5 holds back 1 1/3 stops. Thats easily set with a 1 1/3-stop aperture change or 1 stop of extra time plus opening the aperture by 1/3 stop.
Black and White Filtering Notes ( Nikon EL2 + 55/1.2 AI Nikkor, ASA 100 sunlight, gray card)
This is in the form: Filter, Δf, Δf meter
Hoya K2 (8) yellow, 1, 0
Hoya O(G) orange, 2, 0.6
Vivitar 25A red, 3, 1.6
Hoya X(0) yellow-green, 1.5, 0.5
Hoya X(1), 2, 1.6
The through-the-filter readings are too high in each case. The error varies with the filter color.
Of couse, this problem doesn't affect handheld meters (unless you place a filter between the light and the meter's photocell).