I have a 18.5' WSM and I've smoked from -10F to about 80F and have been able to hold roughly 230 for long periods of time. the only thing I can say is use the minion method, stay consistent with what you like (water in the pan or not), and make very small adjustments to the bottom vents.
leave the top vent wide open all the time, start with the bottom vents about 1/4 the way open. When you make adjustments only go another 1/4 turn and away from any wind if there is any. making major adjustments can affect temp swings..........my wsm doesn't seal up that good and I don't have a gasket kit. in fact mine blew off the deck and has a slight oval shape to it......still works fine......just try to be as consistent as possible with your set up(charcoal, method, amount of charcoal, etc.)
I've learned a ton from this site, and knew nothing a few years ago. the first few times I even made notes from my cooks.........
keep us posted on progress.........
chris
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I also have a little "V" shaped wind break I use to protect the smoker from wind..............I'd guess that is part of the problem