Don't know this camera (have had lots of older Rolleis), but if you want to apply the filter factor to the meter reading (which is in the viewfinder, is this correct?) isn't it possible to apply the factor to the film speed setting, e.g. 3x orange filter, exposing HP5 Plus at a preferred setting of EI 250, reset the meter to a film speed of 80 and leave the filter on the taking lens?
I thought rollei introduced a special filter holder that will let you not just cehck exposure when using coloured filters easily but also use polarisers and graduated filters much more easily?
I am not sure about the GX, but the F has an exposure compensation dial where you can adjust your exposure according to the filter factor of the filter(s) you're using, as long as you are able to remember to return it to zero after you take the filter off...
there is a new ROLLEI TLR filter holder available from Franke&Heidecke, which is just perfect. You screw the filter in and you can swing them from the view finder lens into the exposure taking lens. It's not cheap, but well made, and worth every penny, for the TLR lovers. Unfortunately, it fits only on the newer TLR's, but other sizes for older TLR's are in the pipeline.
I am not sure about the GX, but the F has an exposure compensation dial where you can adjust your exposure according to the filter factor of the filter(s) you're using, as long as you are able to remember to return it to zero after you take the filter off...
If the factor isn't stamped on the filter or stated on a leaflet packed with the filter, you should be able to find it on the relevant website. If it's a genuine Rollei filter, it's probably made by B+W.
Approximately:
Mid-yellow or yellow/green filter 2x
Orange 3x - 4x
Red 8x
Polarizer 3x - 4x
Neutral density Always stated on filter: 2x, 4x, 8x