I have tried 3 methods for larger planes:
1. Put it on the floor and get it level, with an accurate scale under each wheel, and calculate from there (I have a dummy blocks the size of the 1 scale I have, so I can move the scale around one wheel at a time). Worked fine.
2. Build a pair of stands with a fat dowel on top (like 1 inch or fatter). Place the airplane so that the dowels are parallel to the wing spar, so the weight is distributed over a foot long dowel (instead of at a point), but the wing is free to roll forward/backward on top of the dowel. Also worked fine.
3. Build a Vanessa (hanging with plumb bob). Also worked fine.

My preferred method now is (3). It took a while to set up the first time, but now that I did it once it is the fastest and easiest method to set up again (the hook is already screwed into the ceiling, the pulleys and ropes are in a drawer).