Not really sure what heat flux is all about. I don't sub ohm so do I need to know this? Where do I find information about that? I notice it on Steam Engine, but it stays at 10 with the build I check.
In Simple terms, think of Heat Flux as a Measure of How Much heat a Coil is going to give off for a given Wattage. It doesn't matter if you Sub-Ohm or Not. Heat Flux gives you a Numerical Reference you can use to compare builds.
If you fill in all the Fields for a Build you like (Wire Allow, Wire Gauge, Ohms, Coil Diameter, etc.) and then enter the Watts you like it best at, you will get some Heat Flux Value.
Just for Talking, say the Heat Flux is = 200
Now when you change things like Wire Gauge or Ohms, you can compare what type of Watts it is going to take to get to take to get to that 200 Heat Flux Number. And How for Smoke Builds, the Heat Flux Changes Very Fast when the Watts are changed (Low Ohm Builds). And when in other builds (Higher Ohm Builds) the Heat Flux Changes slowly as the Watts change.
Builds where the Heat Flux changes Very Fast for an change in Watts can sometimes be harder to Use. Because the Sweet Spot where is Hits well is a small Wattage Window. Where as when the Heat Flux Changes Slowly, there is a large widow where the Hit taste about the same as the Wattage is Increased/Decrease.
Much of this might not make much Sense right now. And that is OK. Because you haven't had a chance to make many Different Builds. But it can help you down the road as you Branch Out and want to try Different Gauges/Types of Wire.