There are a number of outside threads and videos on fire management that are very good. My advice would be to not be overly concerned about an exact temp. An offset is designed to work within a 30 to 40* range. Mine likes to run at 260 to 275*. When it gets below 260*, I will add a pre-heated split. I always pre-heat my splits on top of the FB. This causes the new split to ignite more rapidly and keep the fire more even. Always try to keep a good bed of coals. The coals produce more heat and the fire just keeps a new supply of coals. If you will begin with a good basket of lump charcoal, that will help to get a good base of good coals, then start adding the wood. With my smoker, I usually keep all of the vents open and control the temp with the fire. Your cooker is a relatively small unit and will be a little harder to learn, but it will come to you. Just keep burning.
I hope you got the convection plate with your Horizon. They are an excellent way of controlling the evenness of the temps in the CC.
Good luck and good smokin', Joe