I think the reason there is no flat base 185 is just that it would be so short in the bearing surface there wouldn't be for the case to hang onto. However, if you pick shapes for which both base forms are available, the length difference will tell you what the equivalent flat base would be if it could exist. Unfortunately, that's little help in finding published flat base equivalent loads when nobody would seat a bullet as shallowly as that would require. You can, however, find other 185-grain load's seating depths and plug the impossibly short one into QuickLOAD to get a sense of the relative effect. If you want to bother, that is.
My take is, knowing this is a hollow base that will blow open to resemble the skirt of a badminton shuttlecock if you have too much muzzle pressure, I would just assume one had to stay with relatively fast powders in small doses to keep the muzzle pressure down, and would just pick target loads in that vein. Fast powders usually manage to burn fairly well even with excessive volume. The old 185-grain target loads of 3.8-4.2 grains of Bullseye or its equivalent in another fast powder would be a place to begin. You could start at 3.8 grains and work up until the muzzle velocity was in the 700-750 fps range and call it good, I think. The pressure will be low at that velocity with that bullet weight; well below hardball starting load pressures. See where, in that range, the accuracy seems best and the gun still functions.
I imagine the idea behind this bullet is to provide a target bullet to those whose guns refuse to feed anything reliably but a round nose profiles.