(I've already have a vetted team to do all the rehab and turnkey work)I initially was offering 10%-12% return on what is provided from the silent investor.

This seems appealing too, but I fear the initial outlay of an Arlington property + ADU would be damn expensive.

In our initial walk though/interview, I asked if there was going to be anyone with him and he said "no, just me" and 3 days before he is scheduled to move in he tells me about his roommate that he's been talking to, "telling her what's up", and her 2 kids. 

So I will not be taking a dividend in the initial years. 

Typically a tax sales is initiated by the County Treasurer to satisfy delinquent real estate taxes and a non tax sale is initiated by a creditor (such as a bank or entity who has the MER lien) who is trying to satisfy a judgment obtained against the debtor.

Set purchase price initially.  

The project overall has been drawn out quite a bit and has cost around double what I had initially estimating, but I definitely take most of the responsibility for that.

That could help even if it doesn't fully answer your initial question, I agree. 

I typically like to start with an initial phone call to discuss your investment goals/experiences, answer any questions you have, and tell you all about what our company has to offer.

You can estimate by calling paint contractors, drywall, plumber,etc and ask what they charge to do a room or by the square foot and applying that when you do your initial walk thru.I divide my costs into hard and soft costs.Hard costs are known costs.