If you are using plastisol ink the important thing is to attain fusion temperature 280-320 degrees Fahrenheit, depending on the type of plastisol ink used.
Plastisol will start to become dry to the touch at 180-250 degrees Fahrenheit, which is what you do when you Print-Flash-Print (PFP) on multi-color jobs that require it.
The thing is to fully cure your ink, it might feel cured but if it doesn't reach fusion temperature it will probably crack, bleed or flake off when the garment is washed.
You might want to try using paper thermometers (heat strips). These heat-sening strips turn black at what ever temperature they reach, they are sold in 6 different heat ranges and are sold by many screen printing suppliers. They also have heat temperature guns that measure the surface tempertature of the ink, they are cheaper in the long run as you don't have to keep buying the strips.
You will never be able to achieve a high production output using a heat gun for curing your final product, you should consider getting a conveyor oven when you need to speed up your production cycle.
I hope this is of some help and good luck on your printing.