The monthly expenses would be around 3,400/mo So subtracting the rents on both original houses leaves me with a $800/mo mortgage on the third new home which I'd live in with my father and split all costs making it extremely affordable for both of us.

This should not be subtracted from your offer because over time they are already included in your operating expenses.

Remember that you should SUBTRACT the cost of tear-down from the value of the property.

You should start at ARV then subtract 60-70% depending on where it’s at in Springfield.

Subtract all damages and then add the unpaid rent. 

Using the calculator on an existing property is a little tricky but the numbers I ran show after paying your expenses and setting aside some funds for vacancy, repairs & maintenance, and capital expenses you're still cash flowing slightly under $100/month (even higher if you don't subtract those reserves).

I search the tax assessor website and found taxes owe on property that I was interned it.If I choose to make an offer I should subtract the balance owe right?

it Is amazing how some people do struggle with basic math.so to me I boil it down to what are doing..are you buying a rentalthat's easy take your rent say 1000 divide by 2 that's 500.. subtract your mortgage payment and that's how much you will generally make over time.. if you do that quick calc and it does not work then pass.. if you do better then great.. but that's kind of what happens .in my hey day of HML  to flippers it was astonishing how many people would send me a deal that went like this.PP  150rehab  40resale  225k Hey I got this deal were I am going to make 35k LOL.. not realizing that this is a break even at best. 

Make sure you run the numbers for what new construction residences are going for, and subtract out construction costs, cost to install utilities, developer profit, etc, and you can get an idea of what a developer would pay for the lots. 

@Oscar BetetaHow much is the home which includes subtracting your down payment, adding closing costs.