I had the same experience as you on my first couple cooks. After that, I found I could control the temp to below 200. Here is my routine:
1. Clean the cooking area
2. Fill the water pan completely full (until it starts to spill over).
3. Fill the charcoal box to the top leaving a small amount of room in the back corner.
4. Fill the back corner with 8 briquettes started in a chimney.
5. Put the charcoal basket in & close the intake to about 1/8, leave the exhaust fully open.
6. Temp stabilizes around 200, slowly open a fraction of an inch to get desired temp.
7. Add wood chunks & meat.
That's it... I burn through a full basket in about 4 hours & use a full 20lbs last cook (1 brisket & 2 racks of ribs).
It takes a long time to heat up with the full water pan and a barely opened damper, but it doesn't overshoot temp. I add water every time I add charcoal. Without water, it's gone as high as 450 degrees.
I don't use a controller, but emailed pitmaker to ask about buying one along with any instructions that may come with a new safe (bought my safe used) and had zero response... If you can't answer email, don't publish an email address.