I do the same on the bottom, but I usually leave the top as closed as it would go. I need to get me a needle valve, definitely would make things alot easier for me, though its not very hard to control my Landmann Smokey Mountain. I definitely need to get plywood though. On christmas eve when I started smoking there was a little wind, and that made it difficult to get up to temp, but once it got up there it was smooth sailing.Well on my propane smoker, I leave the lower two closed as far as they will close, which isn't all the way. I leave my exhaust open all the way when smoking. I have installed a needle valve so I have better control of the fuel. I leave the smoker set to high and use the needle valve to dial in my temps. I can run anywhere from 120/130 to 500+ with little baby sitting. Depending on your smokers set up there are several types of needle valves. Bayou Classic is the brand that most use. Use the search feature here and type in needle valve.
Another thing is that wind effects propane quite a bit. I lean a piece of plywood near the lower vents as a wind break.
I just bought the 43" wide body and noticed that the side vents were designed to never close. Not a fan of this so I switched the shoulder screw used on the vent with one from the water pan rack. Then I moved the vent door to the outside of the smoker. That thing seals airtight now. I also ditched the stock wood box with a cast iron dutch oven that I have drilled holes in the lid. I fill my wood box with water so that when the water boils off, the wood starts to smoke. Plus the dutch oven get much closer to the burners.