The adjustable regulator is used so that you can have the burner on full and still be able to adjust the flow of gas. My setup is a 100lb propane tank connected to a Ransome burner through a ball type gas valve. The gas comes through the regulator which I usually have set at 10 psi. The valve is mostly for shutting the gas flow on or off except the 1st cycle where I use the valve to turn the burner up in gradual steps. From the valve the gas goes through the orifice which is a fixed size(sorry, I don't remember the size and the kiln is not where I am but I can get that later). This sequence determines the BTU value of the burner. 500K BTU sounds like a lot for Raku but that would be a maximum output under ideal conditions so the regulator combined w/the orifice size really determine the output of the burner.
I hope this is not overly detailed. I have used many different Raku kilns and burners but they all work on the same principle.
I will get the orifice size for my burner so we can do the math but this weekend it is camping with the sun-in-laws and the grandchildren so it will be a few days.