As Hugh said "The rule of thumb is that, for astrophotography, you halve whatever the manufacturer rates their mount at."
This also applies to general photography with tripods and ball-heads. Mount 3 Kgs on that you beaut ball-head that is supposed to hold 30Kgs and watch your carefully composed shot slowly disappear from view. While it may well hold 30Kgs in a perfect vertical orientation, as soon as you move away from the vertical, gravitational forces kick in inducing the dreaded 'droop'.
For me, the operative word when selecting any mounting hardware is 'overkill' and this also gives you some future proofing 'coz you know, deep down inside, that you will soon want a bigger lens/scope.
