My first forge was a bean-can fired with mapp. I did accidentally make one forgeweld but didn't find it till much later: now I have no idea how I did it.
my latest forge is a "balloon time" forge fired with a Rex Price Shorty. The sides and rear are lined with 2 inches of kao-wool coated in satanite then itc-100. The bottom is 1-inch firebrick front to rear, with large opening in front small in rear.
I haven't tried to forge weld in it yet, but everything I have worked on, which is to say anything I can fit in or through the cavity, has been worked at a regular bright orange using only 3-4 psi. sometimes as much as 5 when I fill the chamber up.
I have built all of them following directions from here, anvilfire and rex price. I never really did any calculations. I guess I just got lucky:D