Ok, I know how to do the saturation curve to determine what the intensity is per whatever settings....

However what baffles me, is that up till now, the intensity to use per whatever part is being shot seem to be a feel good decision not based on any criteria.

I have yet to see any scholarly, engineer type approach to define if it is this type of metal with this density then the Intensity you want is such and such.

Is there anything out there?

Anything?

I have not heard of the feel good approach but I am familiar with the Xerox engineering approach - copy what someone else did and hope for the best.
Usually there is extensive development work in creating a shot peening recipe (intensity, coverage etc.)
We have many articles on line at shotpeener.com and also have workshops in the US and around the world and also offer on-site training.