Counterweight (ETX-90PE with Canon EOS300D)

I've just bought a T-Mount and adapter ring for my Meade ETX-90PE, to
attach my Canon EOS300D. Overall, the weight of the camera and
mountings is about 0.85kg.

Given that the camera extends quite a way off the back of the
telescope, I'm going to need something more than the 0.85kg weight on
the other end. There's a couple of screw holes underneath where I can
attach a counterbalance - obviously it'll have to have an extension
rod on it. The question is this: Will I need to add more than the
0.85kg in weight on the counterbalance, due to the fact that the
camera extends out so far behind (it's about 5-6 inches)?

The screw mounting is attached to the plastic part of the body and
adding this much weight would worry me. I guess I could always find
some way of getting some weight and using a rubber strap on the end of
the scope to add some extra mass.