All the previous posts are correct and valid. Backstay tension causes the proper tension in the forestay.
If 'poor pointing ability' is the chief problem and you do not have a roller furling mainsail
, there are many trim and sail 'shape' issues that cause 'pointing issues'.
1. That backstay should be tightened to approx. 15-20% tension ... as on most 'recreational/cruising' boat that forestay (reacts with the backstay) should be operating at close to that tension value ... a sailmaker when designing such a sail will 'assume' that the correct tension is in the forestay wire which under normal sailing conditions (10-15KTS) allow the sail to take its proper designed shape.
2. Pointing is dependent on so called 'weather helm' and sail trim. Weather helm is adjusted *primarily* by mainsail luff tension. If you have a 'roller furling mainsail' you cannot do this and therefore must adjust by slight 'reefing' of the main. Otherwise, you must correctly control the main's luff tension ... and then adjust the shape and trim of the boat. Most who use woven dacron mainsails fail to 'properly raise' such sails which leads to poor pointing, aggressive heeling, slow and cranky boat with LOTS of weather helm (rudder being dragged through the water at an angle).
Use a FULL set of tell tales (luff/midcord/leech) plus a row of 'steering tales' on the jib at about 6-8 ft. above the deck; but, FIRST be sure that the mainsail is correctly raised which will affect proper 'shape' for the day's wind and wave venue: How to properly RAISE a woven dacron mainsail - SailboatOwners.com
If you have a roller furling mainsail ... the only way to 'balance' is re-rake the mast or slightly 'reef' the main.