Good Morning, Rusty,
May I assume from your last post that you do NOT have the separate variable sensor which normally accompanies the 365 flash? If not, you are stuck with using it in manual mode. Your estimate is correct; according to the info on my sensor, your E. I. 400 guide number (for feet) should be around 320.
The 365 is a good powerful unit; I normally use mine in manual mode as a supplementary flash to bounce off a ceiling or other reflective surface.