Afraid its impossible to give you an exact loading as it will depend on on how the rig is set up. How stiff the mast is, etc.
Basically you want to get the same loading on the running backstay as you have on the inner forestay to prevent the mast pumping at that point, and trial and error is as good a way of getting the right setting.
Just be sure once you've got it right (on boat sides) you mark the runners so you can repeat the setting easier the next time round.
A useful tip is to use a clear peice of transparent cellphane with a line drawn in back texta pen - that you can hold up and view through to be 100% sure you keep the mast straight.
We've had one lighter weight JOG flyer with a spindly mast, and runners adjusted using a block and tackle type arrangment. Those I just hauled as tight as I could - enough to stop the mast pumping at the forestay / backstay point.
Other heavier cruisers and bigger racers we've owned had their runners led to winches. Just be careful if this is you case as it is easy to overdo the tension with a winch
- especially if electric
. Look up the runner as you are cranking, and stop tightening when the mast looks stable.
Finally don't only use those runners when you are using the staysail on the inner forestay. If you are reaching / beating in heavy seas with the normal forestay in use, the runners can help keep that mast stable. Worth while if you can be bothered.