Some times isn't only a weigh thing. A short zoom (2Kg) plus clamp-on, can do more pry to mount than a heavy wide prime (18mm 5i per example) because the clamp on plus filters is more far of the mount doing more pry. I use a simple rule, if you can unlock the mount without helping the weigh of the lens you are ok. If you feel that you need hold the lens to do more smooth the mount turn when you are unlocking it, probably you need support. Is the same for set the correct high of the bridge on long zooms, attach the zoom and move the high of the support up/down up to open the mount with the same force that if you are taking of the cap