The ratio of IR to visible light varies a *lot* with time of day, weather, latitude, and who knows what else, which makes it pretty much impossible to assign a meaningful EI to this film in general. I've gotten good results at EI 6 on some days, and terrible underexposure at EI 1.5 on others. Bracketing and experimenting to find what works for the conditions you typically shoot is probably the only real way to go.
I've actually had some luck metering *through* the filter, with the meter set to 100. The results were a bit thin and I'd probably add a stop or two if I did it that way again. I have no idea how reliable this method really is, though---I guess it depends on the sensitivity of your particular meter to IR.
-NT