Might be a little late to reply to this, but maybe it will help someone else in the future... I helped a few people weigh taildraggers with the RAA scales, and this is what seems to work best:
1. Weigh the aircraft in three-point attitude. Add together the weight on all three wheels.
2. Raise tail to flying attitude, and weigh the mains. Add these together.
3. Subtract the total (2) from the total (1). The result is the weight on the tail in flying attitude.
You now have the weights necessary to calculate your C-of-G. All you need is your distances from the datums. For a "known" design like an RV, you can pretty much take them from the plans. If you have something else, you'll need to measure them.
This has the benefit of not requiring you to build a support that will hold the tail up and/or the scale up at the same time. You just roll the plane on the scales, note the numbers, then have a stout friend lift the tail, record some more numbers, and you're done. An RV can be weighed in about 10 minutes using this technique.
FYI - Nosewheel aircraft can be weighed using a similar procedure... Weigh all three gear, then pull the scale from under the nosewheel and lower it until you're in flying attitude. If you're lucky, you won't need a hole in the floor for the nosewheel to sink into...