What are you aiming for by rating the film at 800?
Greater flash range?
Do you intend to develop it at 800 ASA?

Anyway, with 400 ASA film, set the 285 dial to 400 ASA, which is its highest.
The dial will show your choices of the 4 auto settings and their corresponding distance ranges: yellow at F4, red F8, blue F16, purple F22.
The flash's sensor will automatically adjust exposure within the distance range shown on the dial.

Or you can fire it at full discharge in manual, and use the corresponding aperture for subject distance.

Those settings would be correct for 400 film. So, if you choose to rate the film at 800, and thus underexpose, you would set the aperture to one stop smaller (bigger number) than the one indicated for the auto setting you choose. Or go 1 stop down according to the manual chart.

If you're just trying to reach as far as possible with the flash, just fire it at full manual power. That's 85 feet at f4 at 400 ASA with the tele head extended. 100 feet at 2.8.

If you're trying to reach further, underexposing portra 400 and push processing doesn't seem necessary to me in any case with that flash.
If it's a stop under due to distance, I'd just bring it up while printing or scanning.
Many labs won't push c41 anyway, and the results aren't great, imo.