The butt has a lot of internal moisture and doesn't need a lot of additional IMO.
A good heavy rub will help build your bark and you don't have to open the door a bunch to mop.
If you really want to mop,wait for several hrs into the cook ,so you don't wash off your rub and the bark you are building.
The Cookshack is a very moist cooker and I don't mop butts ,unless I'm cooking the 8+lb size.
If you are cooking a full cooker of butts,your problem is almost too much moisture.
You can then sorta sling your mop on,to avoid ruining your bark.
I may use a spray bottle with a some apple juice,if I decide to check big butts after 10 hrs, or so .
I find I get best results by adding a little of our rub and a little vinegar style sauce when I actually pull the butts.
As to rubs,there are probably a bunch if you try the search feature at the top of the page.
Basically you are balancing sweet,salt,and heat.
Salt,pepper,sugar are the essentials with a couple of other flavors you might like.
This is just the way we do things and certainly not "the way".
Hope this helps a little.