This mite sound silly but instead of working out how long the bar your using and the weight you need to hang off it why not buy one of those cheap torque wrenches (the one with a poniter on it) and use that! they cost around 10-12 quid.
I know there not great but alot better than try to second guess filling a bottle with water and working out the length of the bar your using it on etc.
This type i'm talking about.