I had heard somewhere that the decision point on max heat gen vs max cool run is 1.37 heat efficiency. If you are lower than this heat gen nodes are more effective, if you are above this then Cool run is more effective. Obviously a combination of the 2 'can' be more appropriate depending on your exact playstyle.
BUT, I've also seen and done math on this and discussed it in detail. At the end of the day the variance in total sustained DPS is in the fractions of a %.
The best starting point is Ash's skill tree recommendations and then experiment for yourself like we did. I dont build my tree's exactly the same as Ash, but I certainly improved them after listening to him, I recommend you do the same.