Temp on the egg is hard to say. Here is what generally happens:
1. get the egg up to 550-600 dome with good burn.
2. drop in pan, and dome temps drops to 300, pan absorbs all the energy.
3. let pan sit in egg to heat up, dome temp slowly climbs as pan heats.
4. usually when the egg dome temp gets back to about 375-400 degrees, time to put meat on as pan is good and hot.
5, sear or blacken, 45-90 sec. depending on cut and pan temp. The flat iron went long. I suggest trimming the fat from the sides of ribeyes.
6. last step is set a grid in the dome (atop the rig) and finish the meat to temp (doneness). By this time the dome (grid) temp is 400 or so, good temp to finish.
I use the guru to run the temps. Typically by the time I get to the finishing stage I can turn the guru off and let the retained heat finish the cook. I leave the griddle in place during the complete cook. I'll throw a small mesquite chunk on the pan for smoke, it almost always burns. After dinner I'll come back out, clean the pan, spray it with Pam, and return to egg to reseason.
I use my Spider piece to hold the pan two inches into the fire ring. The ID of the fire ring is 16 inches. A 14 inch size skillet or pan is a good size as it leaves 1 inch clearance between the pan and fire ring. You need to cut the handle off the pan to fit. I use 16 inch tongs to work in the egg and don't use gloves. The pan on this cook is the 14 inch bayou classic griddle with two tab handles cut off.
http://bayouclassicdepot.com/7414_cast_iron_griddle.htm
I akin this cooking method to a chef searing atop a gas cook top and finishing in an hot oven.
Here is what happens if you blacken ribeyes without trimming the fat off the sides, lots of smoke....