I've not shot this film, but here's how I'd go about trying it out. Guessing your half-moon scene brightness was around LV=-6, and a reciprocity failure correction curve like that of Provia 100F (a big guess), I'd start at 32 minutes for that condition and then bracket down in 1/3-stop increments to half that time, or 16 minutes. Your 18 minute at f/4 test falls within that so this initial guess seems reliable. So for a full moon (LV=-2.5 on a gray card), you should be in the 1 to 1.5 minute range at f/4 for exposure time.
You say the weather was not cooperating when you were running the test. If the moon went behind a cloud for a significant part of the exposure (more than a quarter of it, for instance), your needed exposure time could become much longer, since the clouds can dim the moon (your light source) by several stops. I've had three minute indicated exposure times turn into 30 minute exposures after the moon hid behind a dense cloud bank. Even thin ice clouds can dim the light by more than a stop. If you think this may have been happening the night of your test, you need to test again under stable light conditions.