Stabilizing temp

I load all the hardware once the fire is around soft-ball sized (in one spot) for a low&slow cook.  Shut the dome, place the DFMT set the lower vent to around 1" open then dial it in.  FWIW-

I agree with @GrillSgt that adding the plate setter later does tend to drop the temp for a little while, and I, too, used to put it in at the beginning.

But I hate to have my smoking wood being wasted when the fire's stabilizing, and I don't want to have to pull a hot plate setter out to add the wood when I'm ready.  So these days I start the fire with no plate setter and only a couple of pieces of wood buried in the lump, and let the temperature stabilize.  When I'm ready to cook, I put in a few more pieces of wood, arranged radially around the fire, add the plate setter, add the meat.  I know the temp will drop for a bit, but it comes back up and I feel like I'm maximizing the smoke getting to the meat that way.

Like others have said, though, different people do it differently, and it probably doesn't really matter, much.  Do what makes sense to you and it'll be fine.

I mix wood chunks throughout the lump so they'll smoke eventually. I put one chunk on top of the lit spot, sometimes hit it with the torch too so I can enjoy the smell right away. Once lit, add the platesetter, or whatever. 

Get the fire established, then close the vent(s) to the setting for your target temp. And wait. Takes longer to get to temp, but there's no chance of overshooting if you get sidetracked.

If you are chasing temps it sounds like the fire is not established or you are not letting the egg do its thing. I let my medium come up to 300 then add PS, chunks, grate. Close and wait for the egg to come back to 300. Bottom open and no DW or DW wide open. Once I hit 300 the second time I make little adjustments to get to the cooking temp.