You should use the filter factor of the filter you're using to set your exposure meter by. If your filter requires a 2-stop exposure increase, you set your hand-held meter to ISO 50/18. You can check this by metering a scene then re-metering with the filter over the meter's sensor. If using a camera's TTL meter, set the meter to ISO 200/27; the meter will compensate automatically for the filter. I'd be more inclined to trust the hand-held meter; your mileage may vary.
Originally Posted by f/16
Whatever filter factor you use, you have the lab develop the film normally (ISO 200/27) unless you need to adjust the contrast.
Some useful info here: http://en.wikipedia.org/wiki/Filter_factor