Looks like I dropped the ball on this one
I actually ordered the parts so I could personally test the controller before handing it out to anyone. This is a clever idea to use the commericially available, low-amperage dimmer switch to trigger the high-amperage triac, though.
I think there might be an error on the schematic though. Shouldn't the resistor connect to the other side of the plug?
The triac I ordered was the 30 amp Q6030LH5. You really don't need a 40 amp triac for this, but it won't hurt anything either - and might actually dissipate less heat. The peak current draw will be less than 20 amps for your 1.5 kilowatt burner.
The reason it doesn't start until you turn the knob past 1/3 of the way is due to the 75 ohm series resistor, which has to be there in order to not blast the gate of the triac with the full 120 volts.