Addict, I seriously consider buckie to be one of the world's foremost experts on low-n-slow Weber kettle cooking, so I'm more than a little reluctant to chime in here. But I think you might be over-managing this thing.
In the summertime on a calm day, my kettle runs at 225 with about 5 briquettes to start it, so I'd say you have plenty of fire. Also, remember that this is a slow, g r a d u a l process, so let your kettle do its thing. I wouldn't even check the grill temp for 30 minutes, and I wouldn't expect any adjustment I made to vents, etc. to affect anything for almost that long. Your setup sounds good, so just let it work.
Also, if you're using a non-flip vent, do like the old-timers used to do-- turn the grate until your grate handle is over the coals, and then you can drop fresh coals through the slot.
I would also say that if your grill is coming up to the target temp, be aggressive about choking off the bottom vents to slow it down. It's much easier to tweak things *up* to temp than to try to get a hot fire to burn cooler.
Bottom line: relax. Your kettle was designed to do this. Let it teach you. ;-)