Nozzle sizing root density

[LeakyBucket]

I would like some background and description on the rule of thumb sizing formula (root density?)for nozzles on separators and exchangers. What is the formula? What units go with it? When to use it? Up to now I've used velocity guidelines along with mixed phase density to determine nozzle areas. I've seen people use this "root density" rule of thumb and I want to understand it.