5 to 8 degrees of heel.
If your forestay is loose as it should be, sight up the fore stay when sailing upwind. You’ll see the forestay with the head sail attached is bowed to leeward and forward in shape. The back edge of this headsail, the leech, is curved matching the mainsail and pointed towards the stern clear to the top of the mast.
With a backstay adjuster you’re pulling the top of the mast back, bending it, this straightens the forestay taking out the curve you see. With the front of the head sail straight the back of this sail will twist and not all will point towards the stern. The top will twist out dumping wind, depower the sail, and lower the pressure on the sail to heel the boat, this happens to the leech of the mainsail also.
