Hi,
I'm a pretty inexperienced dirt rider so I can't say too much. I'm running full dirt tires (Bridgestone 403/404) on stock rims and have been using 15psi F&R as a start (used on dry hard pack, rocks, and mud, no sand). If you're on the stock dual-sport tires that's another matter but you'll probably be in a similar range. I recommend talking to ICE-riding dirt bikers as the tires don't care much what the powertrain is and the FX weighs similar to many dirt bikes.
What I really need to do is soften up the suspension from the stock settings, it's too stiff for the street, much less when the rear gets kicked into the air by small jumps off road. Once I get that dialed in I'll start fiddling with tire pressure. Just got my second set of batteries yesterday, looking forward to doing a whole weekend in the wilderness with no charging!
Also, I've always ridden with both batteries in (need it for commuting and just haven't tried it yet offroad), but talking to some folks who ride dirt at the factory yesterday while getting my recall done there was a split between front and rear for one battery. The one who rode with it in the front did serious big-air motorcross and liked that it helped keep the front tire down. I'm more likely to try it in the rearward position as I prefer single track and moderate speed trails with only the occasional tentative foray into the air and I think that will keep it closer to the CG for better response and balance.