I noticed that holding the desired temperature got easier as my WSM accumulated a layer of seasoning (crud) on the inside. It bounced-around a fair amount when it was new and was MUCH more sensitive to vent settings than it is now.
SO - I also use the modified Minion start (I usually use Lump) with a PACKED ring of un-lit with smoke wood mixed-in.
Then (ALL Vents 100% to 3/4 open) I dump a FULL Weber chimney of fully-engaged lit on top, assemble the cooker, fill the water dish with HOT water, and put the meat on.
After about a half-hour and hour later, I check the temperature and adjust the vents accordingly. I usually catch the temperature on the way up.
Usually, from then it's smooth sailing for about 6-hours, when I sometimes need to stir the coals and check the water pan.
I also don't lose sleep over about 25-30 degrees. I find that once it gets in the zone, the less poking and prodding that I do; the better my cooks usually turn out. If it's running a little hot, I'll try to adjust the vents down a bit but if it's close to what I want - I leave it the heck alone!