Ordinarily, you would want the top vent all open so whatever fire source is going will exhaust cleanly.......
Two things I'm sure of.....1) weber's got the amount of exhaust figured out 'cause it works perfectly most of the time......2) I've lived the choked down, ill burning fire enough times to know the difference........
Whatever amount of airflow in, it has to have 'enough' of an escape route to get a continual feed of air thru the cooker............the people that build big guns trailer mounted cookers are all over this thing called "DRAFT"( the draw of air thru the chambers & out the stack, ex: if the stack is too tall for the volume of the chamber, the air/smoke will stall & not flow thru...)................................if the air 'stalls' inside the kettle, you're much more likely to get stale smoke, creosote & possibly a bad taste in the food...............
All that said, if you already have a good, hot clean burning pile of coals / wood, then you can make moves on the top vent as long as you don't overdo it.......I did this last nite cooking a batch of 'turds & some chick br.......the coal bed was way hot & after the chook was done, I pinched down the top vent to lower the heat for finishing the 'turds.....worked fine
How much is too much?
Depends on how much coal you have going & how far above the target temp you might be & for how long things need to keep cooking......
what I do is practice, practice, practice & eat the results in between each........