Here's a useless rule of thumb for you.

Balance the plane (empty fuel tank, of course). Check COG again with a full tank. That range is how much your COG shifts in a long flight. Divide that range by four and that figure is how much you should incrementally change the COG by when making adjustments.

I just made that up, but it sounds good. I would recommend 1/4" increments. It really depends on a lot of things. Percent of chord, aerofoil shape, weight of plane (wing loading), your comfort level of instability. Some designs have a wide "sweet" range, others don't.