If the lot is in a neighborhood, and its only use would be a SFR, you need to determine what the home would be worth if built to the same standards of the existing homes, and subtract build costs.

I used comps and then deducted for repairs subtracted my profit and holding transaction costs then bam, I had an offer amount.

ADU additions are typically value subtracts (I.e. they add less value than they cost to add).- ADUs have no income until rented, but there is a cost from the moment you start the process. 

I typically subtract 3%, of the ARV.Realtor Fees: What is the commission you are willing to pay your listing agent (unless you are the listing agent) and the buyer's agent.

My thinking was maybe a 50/50 split in the ownership in the company with the initial investment subtracted first.

Once you subtract property management ($1,080/year), vacancy ($1,200/year), capex ($220/month), maintenance ($50/month), and insurance ($50/month) you're left with about $300/month in cash flow BEFORE the mortgage payment.

The cash flow is yours to do with what you please ... just saying you can use the cash out refinance as a mechanism for managing that cash flow and keep it in the "sweet spot" where you show a loss on paper after subtracting the depreciation expense, but you still put some money in your pocket (or redeployed wherever else) every month before depreciation.

Now subtract all expenses including the number you come up with for a years worth of rent and $22,000 from your annual income.

I typically subtract 3%, of the ARV.Realtor Fees: What is the commission you are willing to pay your listing agent (unless you are the listing agent) and the buyer's agent.

Here's one thing to consider-- you're limiting your buying power and options by subtracting flip profits from your total acquisition funds.