I'm no expert, but after some trial I set the temp to 20 degrees c above the solder paste temp and leave the flow low. Remains to be seen if I'll damage the gun that way, the temp is at the low end of the gun's range so...I'm guessing it should last. If not, oh well... another fee paid towards learning. $40 gun off of Amazon.

I've done some test work with the pre heater and without, with good results either way...I reckon the potential warping of a circuit board goes hand in hand with the size.