Agreed with all the above........top vent is best if you are able to keep it wide open.......the fire needs to exhaust cleanly.......
Set the temp by way of the intake / bottom vents, likely what people are saying, just slightly open should do......
Two things about this:
1) closing off the top vent can lead to stalled or stale smoke in the cooker
....and,
2) Attempting to force the temp down to very low can also lead to a sluggish fire.......a second part of this can be if you're getting temp from a lid therm, it's reading higher than what's going on at grate or Meat Level (<< Technical Term) so 225 at dome might be as low as 190 at the grate, leading to a "forever" cook & we just don't want that......
A lot of items, I shoot for 260~275 & that seems to be a good median temp fr many cooks, ribs, beef ribs, any kinds of sausages or fattys, ABT's, pork butt & brisket / clod larger beef roast type things.....a little low for chicken maybe, but good for many items.....
It's usually best to keep the smallest clean burning active fire area as possible to get to the temp you want......heating the cooker thoroughly at the start is also a good idea, usually 20 minutes will do.....