Running the numbers simply means calculating the income and subtracting the expenses.The fancy term for this is underwriting but essentially you're trying to estimate how much you will make from this property.You'll want to estimate- Taxes- Insurance- Maintenance- Utilitiesthen subtract them from your potential income

However, since this was used on your investment properties you can subtract it off any rental income you had.

You'll probably have to subtract from that gain some negative cash flow (10% down may make cash flow off the bat tough), and you can add some back in for tax benefits and debt reduction, but the wealth on your balance sheet grows fastest this way.This is not to say 10% down and negative cash flow is good. 

Note this does not mean I make the full rehab cost because holding costs have to be subtracted.  

Wondering, based on this overview of our strategy, what would you add or subtract to optimize our time?

Its more work up front and it requires me be constantly adding and subtracting but I have more control of my contractors that way.

In the absence of either of those, I would use SFH and subtract for the smaller yard, reduced privacy, etc that impacts your ADU.By the way ADUs in southern CA are often one of the worse RE investments.  

I think you’re trying to overpay taxes…If it’s worth $650k you won’t get taxed on $650k unless you bought it for less than $1.Otherwise, take the value today, say $650k, subtract about $50k for selling costs.

Your principle and interest can be cross checked on an outside calculator (Google: mortgage  calculator) and it will pull up a super simple one that Google has... but notice the two spaces between mortgage and calculator... it helps get you to it quicker, instead of the website named that.If your figures match what the google calculator gives you, and then the subtractions are $200, you know you are doing it right.But I will suggest that it's just that there are very few deals that work for investors at 7% interest range right now.  

I want to subtract the amount for what it would take to get the job done properly and pay him the difference.