You cannot calculate rent by subtracting off the rent of the ADU.  

I would calculate this as a 6 month period of interest to investor to be safe. 3.

What worked for him.. and could work for you.. is taking a calculated approach by "house hacking."

Utilize tools like BiggerPockets for resources and calculators, and consider creating a simple spreadsheet to track property details, budgets, and timelines.

As its interest is actually calculated daily.

If a sponsor is touting a 7.3 entry cap in a 6.4 cap market, I'd be digging really deep into how that 7.3 was calculated, and how that 6.4 was substantiated. 

SDE is calculated by taking the net income and then adding back certain expenses, such as the owner’s salary and non-cash expenses like depreciation and amortization.

Many times their cash flow calculation is using flawed underwriting.  

That being said, anything can happen, but if you run your numbers accordingly, make sure that your contractor is trusted and do all the other stuff you'll be a little more protected than the next person flipping that does not do this type of calculation. 

Here's the breakdown of rental income and expense analysis:1.Annual Gross Income: $25,800 (Monthly rent of $2,150 x 12)2.Annual Expenses: $25,800 * 0.40 = $10,3203.Annual NOI: $25,800 - $10,320 = $15,4804.Annual Debt Service: $10,680 (Calculated previously using a mortgage calculator with a loan of $131,775, 7.25% interest, and a 30-year term)5.DSCR: $15,480 / $10,680 = 1.45 (approximately)