Put half the time in each slot, since presumably you're playing half as many tables of each type as you usually do. That is what I have done.

EDIT: Or, I guess, in your case, 1/3 of the minutes in PLO8 and 2/3 in PLO.

PT is not compatible with many sites, so I can't use it to keep good track of a significant % of my play. But if all the sites you play are PT-compatible, then I have to second what Stone & Kuso said.