I put this over in the beginners forum but it seemed a bit pointless there as it really applies to larger planes, although it can be used on smaller ones too. My wife bought me a building stand for my birthday this year.

http://www.jlproducts.net/ProductSkyJack.html

The stand is really nice and very sturdy. One of the problems I've had since moving to 30% was how to check the CG. Couldn't do it on the small Great Planes CG Machine for glo planes as the gassers are way too big. Here's my solution. I pulled the top end caps out of the two front posts and put in a swivel tool post screwed onto a .500-13UNC set screw 1.5" long and wrapped a couple layers of duct tape around them for a snug fit. Then I disassembled the old Great Planes stand and installed the scales onto this one. The posts adjust out and in for any width. Works great!!!! When not in use, just tuck the scales into the padding for easy storage.

I and several friends use this one; http://www.rcuniverse.com/forum/m_708060/tm.htm
have used it to balance a Don Smith Cionnie, Comp ARF Mustang, Ziroli Stuka & Zero. 1/4 scale P-39, 1/5 Mustang, 1/6 T-6, 1/3 Pup, and many others. Works really well, and if you use a pulley at the top, you can do heavy models by yourself.

Here are pictures of my stand which is similar to what you have. So what do you do, put the wings on and place the plane on the swivel at the CG point? Pardon my stupidity but I've spent fifteen minutes studying the pictures and still not sure what you are doing. By the way, my stand comes out of the base and the spikes on the bottom are then pushed into the ground at the field to hold the model while it is being cranked.

Yes, I put the wings and hatch on the plane, then insert the scales into the slides, and sit the plane on the swivels at the point where it balances. Then I can slide the scales in and measure exactly where the CG is. Works the same as the original Great Planes CG machine but on a larger scale.