First Boston Butt on a Brinkman Smoki n' Pit

Welcome aboard.  I don't use cowboy lump or a brinkman so can't help with determining the amounts you'll need and how to get temps stable. ...but you'll figure it out and others with your setup will offer advice. 

Your first few smokes are going to be experiments and learning will be the key.  It's like making biscuits.  Sounds easy but until you do it a bazillion times you don't know what to expect.  You're probably not going to meet your expectations the first few times.  But you can still eat your mistakes!

My tip: don't over complicate it.  It's really only temp control, taste, and time.  Master temp control first by learning to build a long slow burning fire, setting vents, learning when to add fuel for maintenance, etc.  Without temp control your outcome will be inconsistant.

After temp control keep a log of what you started with and added when (rub, mop, marinade, sauce, foil wrap, etc).  That way you can duplicate it once you find out what you like.  Believe me you won't remember four smokes ago what you did to get that certain taste you liked.  I recommend starting with published recipes as your departure point.  Those recipes are tried and true from those who've gone before you.

After that it's just planning the time it takes to prep (can and will be days prior), setup, cook, rest, etc.  I recommend only cooking one thing at a time at first (say ribs, or chicken, or butt) before doing two (butt and ribs).  Doing two at once with different things going on at different times can get complex.  When you do start doing two or more things at once (and you will) I recommend what I call a smoke plan where I backwards plan all the major steps so I know what to do when.

It gets easier once you're confident in what to expect. ...but keep it simple at first.

Thank you for posting and asking, we're here to help, and I'm sure there will be folks who have that unit who can answer you.  However, as your question relates more to the smoker than the smoked, I will move this into the charcoal forum for you for better chance of getting your questions answered for your next smoke.

One trick I've found is that when you add more charcoal, add it already lit. Start a chimney when your temps start to fall and when the chimney is good and lit, then add it to the firebox. This will help you manage a more even bed of coals. 

Bama is correct. You'll need to learn the smoker like any other piece of equipment. Plus the more you use it, the more it's going to seal up little cracks that will let in unwanted air. 

Hope this helps. 

Oh and for what it's worth, I've used about 20#'s of lump for a 14 hr smoke. 

Welocme to the forum! Maybe this will help

http://www.smokingmeatforums.com/t/75110/horizonal-offset-smoker-mods

Welcome. I used cowboy one time and it was bad. My wal-mart here carries Royal oak and I love it. It burns good and long and keeps good temps. Good luck and post some pics.