Equally Splitting the Bill: One straightforward approach could be to divide the total water bill evenly between the two units each month.

Are you looking for something that is already divided or are you open to the idea of finding a SFH with the right layout to divide it yourself?

Let's divide that number by .75 = 413k. 

Take that number and multiply it by 12% and that’s your allowance for marketing per deal  Take that number and divide it by your historical lead cost.  

If so, is the gain simply divided to 50:50 between 2 units?

My assumptions based off research is I will be able to deduct 1/2 of all allowable expenses based off the ADU square footage (My understanding of that list below)  1/2 Mortgage Interest (Not principle) 1/2 Utilities 1/2 Property Taxes 1/2 Insurance (assuming I don't have separate ADU insurance)  1/2 general house maintenance (depending on the issue and if it's related to the ADU)  1/2 Depreciation (Home value, not land divided by 27.5)  KEY QUESTION: If my mortgage (PITI) is $5400, and all of those ADU deductions equal $3800 monthly, and my ADU income is $1800 monthly, am I allowed to pay the difference in "deductions" from my real estate business income? 

This is your annual net operating income (NOI) divided by the purchase price.

@Peter McCauleyYou will have to record and divide your expenses using rental square feet vs. personal square feet or rental bedrooms vs. personal bedrooms.

Your ROI assumption is that when you divide the lines, the tenants will pay for it.

Bonus: Find homes that can be easily divided into 2 units (e.g. main house upstairs with separate downstairs area including bedroom, bathoom, wet bar and separate entrance).