On the road TP can be worked out by getting a loaded as you run it weight on each axle or wheel if you like. You can then look up the TP for that weight on a tire company chart. You can find them on most major tire company sites.
You are right you will have to really watch the rear tires when you lower the pressure. But I would think you will find a tire pressure that works. Remember you have two rear tires on each side so have a larger area and will not need to drop the pressure as much as the front..