Its easier to do this than ask on line. Expose half a sheet to a uniform target at 100 or 80 or whatever in your camera (darkslide half in). Process the negative. Place the clear part of the negative over your meter and note the reading. Slide the exposed part over the meter; if the meter reading drop by one-third, you EI guess should yield acceptable negatives in most situations.
I've had good results exposing it at 50 and developing in HC-110 using the times for 100 but shaving off about 10% because I use a water stop bath and to tame the highlights.