When I am cooking on a Minion method longer cook, here is the way I set up.
Fill the charcoal ring with charcoal, make a dip in the middle, light a chimney load of charcoal, and wait till it is beginning to ash over, (10 – 15 minutes), pour the lit charcoal into the dip in the middle of the unlit, I now put on the center section, and leave it alone for another 10 – 15 minutes.
I assemble the rest of the WSM, putting in the water pan and filling it with boiling water straight away, bottom rack, meat on the bottom rack, top rack, meat on the top rack, lid.
I have a thermometer on the lid and it normally climbs to at least 255° F, and most often up to 265° F - 270° F in the first 10 - 15 minutes after everything is assembled. The times are all walk around estimates I don’t go set my watch and do it all bang on the minute, so it is somewhere between 40 and 55 minutes allowing for fuddle factor.
The temperature that the meat is actually going in at is hard to judge, but I would think it must be going into the assemble mix somewhere in the 240° F – 260° F zone, otherwise it would not be reading 255° F + at about 10 –15 minutes into the cook. The cool meat must cause a drop in the temperature of the cooker, IMO.
I forgot to say I normally cook ribs over something else, a brisket or a couple of butts so the ribs going on the top rack are the last things in and the first things out.