What are the "rules of thumb" for self-storage

[Nicole Heasley Beitenman]

I may have an opportunity to aquire a small self-storage company with about 32 units. I know all the rules of thumb for residential real estate--the 1 or 2% rules for cash flow; the 50% rule for expenses; 5-10% for vacancy, capex, and property management each; $100 per door for cash flow; etc. When evaluating self-storage units, how do you know what the "rules" are? 

I think capex will be easy to figure out once we tour the property and have the roof, driveway, and building inspected. I expect repairs to be lower than residential because, theoretically, renters won't be going in and out of these units often. I expect a broken door here or there, the occasional dent, maybe the occasional weird instance of someone causing damage by storing something weird. I'd probably keep vacancy the same as residential at 10%. 

But how do you determine what "good" cash flow is for self-storage?