To figure out how much counterweight is needed to balance a particular scope you can calculate the torque that the scope exerts. You will need to estimate where the center of gravity for your scope and attachments is located. Then measure from that point to the axis of RA rotation. Once you have the distance, multiply that times the weight of the scope and accessories.
The illustration above shows an imaginary SCT that weighs 30 lbs. with its center of gravity 14.5" from the RA axis of rotation. 14.5" x 30 lbs = 435 inlbs. of torque. Next figure where you want the center of gravity of the weights. Because of the nature of moment arm there is an advantage to having more weight closer to the axis rotation than less weight further out. The location in the illustration allows room for adjustment and for adding heavier eyepieces, cameras, etc. So, figuring the distance of 10" from RA rotation we would divide the torque of the scope by the distance to the weights. The result would be the total required weight. 435 inlbs / 10" = 43.5 lbs. Two 21.7 lb. weights would suit this situation. Or, you could use three 14.5 lb weights. To figure the center of gravity of a set of weights you need the length of each weight. AP3GT1HD22lb and L3G1123lb are 3" long. AP2GT1HD14lb and L2G1115lb are 2" long. AP1GT1HD7lb and L1G117lb are 1" long. AstroPhysics 900GTO (approximate measurements) Losmandy G11 (approximate measurements) One of my saddles, a DC11 dovetail bar, and a radius block for mounting on a C11 all total about 1.7".Add to that the radius of the scope (if the center of gravity is in the center of the scope). 
With a piggybacked scope you would add the inchlbs of the guide scope to the inchlbs. of the main scope. If the main OTA weighs 15 lbs. and the guidescope weighs 7 lbs. the calculation would go something like this:
(Main OTA 15 lbs. x 10" = 150 inchlbs.) + (Guidescope 7 lbs. x 17" = 119 inchlbs.) = Notice that the guidescope is less than 50% the weight of the main OTA yet it requires about 80% as much counterweight as the main OTA. There are several combinations of weights that could be used in this example. 1. Two AP214lb weights centered about 9" from the axis of rotaition. 2. One AP322lb centered about 12" from the axis of rotaition. 3. One AP214lb and one AP17lb centered about 12" from the axis of rotaition. 
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AstroPhysics  G11 

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