I have a stenner setup, but if I had a SWG I would set the level to whatever maintains my FC at the target level without having to add other chlorine products. Though FC loss will usually go up and down with changing weather and swimmer load, you should be able to determine what level works best over time to maintain your target FC. When in doubt, err on the side of slightly higher FC. If you find your FC continuously drifting up over your target level, then it makes sense to dial down the SWG a bit. Eventually you will find the right setting for current seasonal conditions.