RE: Finding CG on Biplane
Actually, that link shows the Vanessa rig. It's a fine rig, and I have no quarrel with it, other than it's just a bit more complicated than it needs to be. My 'sling' is two lengths of nylon strap from Home Depot, which come with buckle to adjust the length of the loop. These are connected by a short length of nylon cord (nylon slides easily over the skyhook). Slip the loops around the model, either fuselage or wing, just so they surround the CG. Hoist the loops and hang everything by the nylon cord connector over the skyhook. You should have it adjusted so the model is just a couple inches over the ground/floor/table, just for peace of mind that if anything goes wrong the model doesn't have far to fall. (Never happens, but peace of mind is nice.)
So now your model is swinging in the breeze. Slip and slide the model inside the loops, and maybe the loops over the hook, so the plane is straight and level. If it looks straight and level, that's close enough. No spirit level needed.
Your plumb bob needs to hang from the same skyhook by a cord with a slipknot, so you can slide it down to where the pointer is very close, like a quarter or half inch from the model. On the model you will have placed a piece of masking tape or whatever, on which you have marked a little 'X' for your desired CG. When everything stops swinging, if the pointer is directly over the X, you're golden! If not, put a weight somewhere on the light end of the model, and it will swing the X closer to the plumb bob, pretty much instantaneously. Steady the swinging, and see how close you've come. If you went too far, move your weight closer to the CG. If not far enough, move the weight away from the CG, or increase the weight.
'The weight' is maybe your battery, which you've saved as the last component to install. If the battery is not heavy enough, consider using a bigger pack, or dual batteries. Or if you don't want to fool with it any more, just use lead. You can assess the fine tuning of CG by using wood vs CF prop, spinner or no, heavy or lightened backplate on spinner, light or heavy tailwheel, move servo from radio compartment to tail or vice versa. I often find myself putting the battery somewhere along the tail of the fuselage, which means cutting a hatch. That's what happens when you 'over-power' with a big engine, or succeed beyond your expectations with keeping the tail light in construction.
The Vanessa essentially does all the above, but with a few refinements--which are nice, but not necessary. The twisting of the dowel is a fancy way to adjust the model so it's hanging straight & level. I find it easier to just ease the weight of the model with two hands and rotate it in the sling to where I want it. The Vanessa shows lengths of cord around the model. Cord works fine for smaller models, but for bigger planes the nylon strap avoids any tendency to cut into a wing trailing edge, or otherwise bruise tender balsa.