1.7 stops would be the minimum exposure change for your filter. Depending on how you rotate it and the scene it could easily drop the light another 3 stops or more. All depends on the polarization of the light before it tries to pass thru the filter. This is where ttl metering really helps. There are some very good pol filters made where the extinction of crossed pol filters is nearly 100% of visible light, but they are not usually sold for general photography. The effectiveness of pol filters for general photography vary quite a bit, the usual indication of effectiveness is the brand and retail price.

Metering with an external reflected or incident meter just isn't going to work very well. You could get a second filter and mount it in front of a reflected light meter and set the rotation angle to match the one on your camera, but it is a pain and you would really need matching filters with degree markings on the perimeter for it to work very well.

End of ramble.