Get a pyrometer from auberins or even harbour freight. You will need 10 firebricks to make a minimally serviceable forge. I used one temporarily, with 1/4" steel on the floor above the fire brick, and I could get a good vortex with careful torch placement. It was finicky, but once I got the angles and placement of the brick worked out, I could hold temps within +/- 5*f. It's not the easiest way to go though, and is the minimum that will work with anything that needs a soak time of more than two minutes.