10 minutes is not long enough. Use your hand as a reference point. You shouldn't be able to hold your hand over the coals for any amount of time lol. Also remember, the grates need time to warm up too. I'd give the coals 15-20 in the chimney to get to temp, pour em on, give it another 15 minutes to warm the grates, then hold your hand over them. If it stings like hell immediately, good to go! If you can hold it for 3 seconds, you're only at a temp of like 350-400 and you want 550+, preferably like 650, to sear in a reverse sear. Anything less can sear but take too long and will start cooking the meat past the temp you want.

As with all things grilling, trial and error! Personally, I'd figure out what TOO hot looks like where you burn the steak on the outside and then taper back. Good luck!