Like most others around here i find closing the top te wrong thing to do more often than not. Only when I'm trying to drastically drop the temp do I close the top vent.
For the bottom vents, I saw a tip on this board some time ago but I don't have a thread link. While the lid is off, and with the vents closed to the far left, open your vent 1/4, 1/2, 3/4, and wide open. At each opening, use a sharpie to mark it. You'll find these marks become spaced disproportionately wider as you go wider open and that'll be okay. And as you use your grill you'll find the handle to open the grill becomes a bit looser and therefore the marks more approximate, but it'll give you more control than you have now to know where you're at.