I watch the vapor cloud build up in the trench before taking my first draw. I then let the next vapor cloud build up before taking my second draw. With practice you should be able to avoid combustion.
When I can't look at vapor cloud build up, I go by seconds. I wait 2-5 seconds depending on power source, draw, wait another 2-5 seconds, draw, and repeat.
With a fresh 2400, I don't 4 seconds.
With a fresh 2700, I don't go past 3 seconds
With the PA, it varies depending on the power I feel like using
I count in alligators to make sure I am not going too fast or slow. With practice, you will eventually find your technique.