HI Bundeenaboy,
In the old IOR era, the standard response when racing was to wind on the backstay till the hatches started jamming from the boat being bent like a banana .
So it can take a lot more than you think. In medium to fresh winds, the amount of pressure in the sails will be pulling your mast out of shape, so give it a bit more than you think you should. Go forward and stand next to the mast and look up as a guide to whether you need more or less. If your mast is bending to the point the midspan of your mainsail is too flat, ease it off.
The other easy way to set the tension is when going to windward, look at your headsail. If the forestay is curved to leeward, you need more backstay. Keep going until the forestay is tensioned properly and nice and straight. You can't point with a soft forestay.

Have a play without the sails up.

Wind a bit of runner on to the point you're happy with your inner forestay tension and the sight up the side of the mast.
Your mast should still have some forward (middle forward) prebend, about 1/3 the mast depth.
If the mast is too straight with the runners on then you might need to tighten up on the inner forestay and forward lowers while loosening the aft lowers a bit.

shaggybaxter said..
Keep going until the forestay is tensioned properly and nice and straight. You can't point with a soft forestay.

Depends on the boat and the conditions. Most top boats in many classes ease the rig tension to soften up the forestay in light winds. Classes like Etchells have a pretty soft forestay in all conditions and still point well.