Hi DJ.
What pressure Propane will you plan to use?
I would use 15 or 20psi typically, via a standard regulator, Which I think should be used with a 0.25 mm (No 8) jet, 2 x 3/16" air holes, into a 6mm / 1/4" mixer tube... By using a variable regulator I can go down to 5psi or up to 30 psi gas pressure, which gives very good adjustment of the power of the boiler to steadily maintain steam without blowing-off the safety. Otherwise you need a very good gas control valve to adjust the burner, but you don't know what the right setting is! When making a set-up nowadays, I prefer to actually have a base board, so all the gas control valves, pressure gauge - or whatever I want to use - is on the board permanently connected to the boiler. Then I only need to connect the gas hose from the Cylinder onto a connection on the board, so everything - including the tall and not so stable boiler - is safer to use on the bench. (I'll get a photo).
I have made a boiler burner (4" diameter) and set it for gas at 37mbar from a camping burner regulator, but then need a both stop valve on the propane cylinder, and another valve at the burner end, to adjust the supply and turn ON/OFF. - If that is what you have for barbecues, etc. and want to go that way, you'll need a 1/2" bore mixer tube, at least 3 1/2" from air holes to burner body, I think 2 x 3/8" air holes, and the gas jet will need to be 0.40 or 0.45mm. I shall need to make a mock-up and check, as the low pressure and small burner is off my table of gas power vs. jet size.
Please wait until I can confirm correct sizes, but please tell me what Propane pressure you will use? As this is all critical, I really want to prove it myself before you set fire to everything!
Don't rush, be safe.
Regards,
K2