I'm having trouble putting a conservative dollar amount on the cost per square foot to use in my calculations before making offers.

The ADU is a separate building from the main house, but is the same address (unlike a duplex).When you calculate your proration for expenses, do you consider un-rented days as personal use days?

When I calculate BRRRR viability, I don't even think about COC.   

You Dad is going to be your best bet here.

Investors can’t eliminate market shocks, but they can hedge their bets against booms and busts with a diversified portfolio and strategy based on general market conditions.

The property is currently occupied and has longterm tenants in it, how can I get the information needed from them to finish calculating the deal? 

Try the BP rental property calculator to see how the numbers break down in addition to analyzing all future investment properties.

I calculate out of pocket costs with downpayment and neccesary repairs to be $75,0000. 

But they have everything online now, and further checking indicated my water usage per month was calculated as 344 gallons per day for the property. 

If you plan to buy and hold after you PCS to your next duty station, then it will be up to you to run the numbers before purchasing to ensure that the property will + cash flow once you rent (use the BP calculators or get a sanity check from BP nation to assist).