One way to guide a roof raise is by optimizing for cost. As an example, sheet metal is universally available in 4x8 feet, but depending on distributor may also be available in 10 or 12 foot lengths and 5 or 6 foot widths in select gauges and finishes. If a 1 inch lift consumed half a sheet of metal and the other half went to waste, it would be better cost-optimized to do a 2 inch lift and use the whole sheet since it's paid for either way.
Competing with that is "I don't want it to be absurdly tall, even if the extra height is free."
In my case I had a specific design goal: enough height to fit a 3-level bunk bed over the rear wheel wells. I made a mock-up in the garage with adjustable warehouse steel shelving considering the thickness of a mattress, the framework to support it, and how much space I felt was necessary to be able to wiggle, roll over, and climb in and out of the upper bunks comfortably. I settled on a 16 inch raise yielding 13 inches of new interior height (allowing 3 inches for floor and ceiling insulation and finishes) to make room for those bunks. That should work out to an 88 inch center-of-aisle finished interior height.
It would have been "free" to go as much as a foot higher based on material sizes, but I opted not to in favor of keeping the top of the bus out of low-hanging trees and keeping the wind profile just a little lower.