I'm my 14" WSM I too only light about 8 coals initially, then watch it over the next 15 or 20 minutes as it comes up to temp.
I'm at sea level in Florida, so my settings may not work for anyone else, but I've got an old tine that broke off of a grate that I use as a guide for the largest opening I want. It's maybe 1/8"diameter. Insert steel part and close the vent snug to it, then slide it out. This gets me right about 300-325°F.
If I'm going for actual smoking temps, I've got a piece of bailing wire that I doubled over on the end, and I'll use that as my guide for low temps. Same process, insert the metal into the vent and close the vent down on it, then remove the wire.
Different cookers will have different levels of seal vs leaks, so experiment with yours. And my own settings don't always work consistently. As others have said there are lots of environmental variables to account for.
Enjoy!