One thing that might help...
Don't use the SWG output setting to move your FC level. That'll send you into bouncing-ball mode! Which might be what you're experiencing. Here's what I mean:
Say the SWG output is set to 40%, your FC is 4 and it should be 6. Set your output some reasonable amount higher (say 45 or 50%) and then manually add enough chlorine to get your pool to FC 6, right away. Then next day see if you're still at 6. Don't try to use the SWG to move your FC from 4 to 6, because that'll only tell you what output you need to move FC, not the number you're really after, which is the number you need to maintain FC at 6.
Conversely, say your output is 40%, your FC is 6 and it should be 4. Turn off your SWG, let the pool's FC drop to 4, then turn your SWG back on, set it to 35% (some reasonable amount lower). Then next day see if you're still at 4. Now you know 35% maintains a FC of 4. With any luck (and eventually with experience), you'll hit it with the first adjustment. Otherwise, you use one or the other of those two processes to dial it in.
It's easier to do than to explain!
Like Jim, I run my FC a point or two high. I like that extra buffer, which comes in especially handy during the two "adjustment seasons" (spring and fall). If your adjustment undershoots, then you've got a little buffer, to keep you well away from your minimum FC. You add a little liquid chlorine to reestablish the target FC level, then try adjusting again.
