If you want to move to having an adjustable backstay, then you want to be able to slack your backstay almost completely off. You first want to setup your prebend, as Jackdaw notes, with your shrouds. How much depends somewhat on the condition and cut of your main and the rig. With your backstay slack, the amount of pre-bend should be enough to provide maximum draft (light wind conditions). Then, when you're sailing, and you approach/exceed your design wind, you will apply progressively more backstay to de-power the main to keep the boat on its feet ( too much heel is slow)
If you are unable to loosen the backstay to the point of being slack, and especially if it's forcing bend into the rig, then you may need to add a extra toggle to one or both of the backstay turnbuckles. It's possible your forestay needs to be loosened raking the mast back, and thus giving you some slack on the backstay, but you should really adjust your mast rake in order to provide the correct helm balance, and then worry about your backstay length.
Adding the adjuster that I described will require a little more slack in the backstay because the johnston adjuster typically pulls the split stays together just a little, even when completely eased off.
Chris
