Start with the potential gross income then subtract the operating costs to determine NOI, divide that by the CAP rate in your area to give you the value, then subtract development costs, building costs, commissions and interest expense.

The bank will subtract around half of the rent amount from your mortgage amount which will allow you to qualify for a bigger loan (because you will have a better debt to income ratio). 

When looking at the trailing expenses, you add back in the trailing property tax expense, then subtract out the expected new property tax expense (under the principle that the taxes will adjust after the sale to reflect a higher assessed valuation).  

If you subtract the land value from the purchase price that will give you an approximation. 

Either for a small gain, thereby adding to your income, or a small loss, subtracting from your income.

I think an incredible idea is simply the concept of a podcast where you literally are sitting in your first calculus class with only a knowledge of adding, subtracting, multiplication, and division.

Supposing I get the average rent per room this way, I could either just multiply by 4 to estimate rent for the 4/2 condo, or I could subtract some on the assumption that the average student bedroom rents for more than each of mine will.

Then subtract out all the costs for running the property (vacancy, trash, utilities, management, etc. but not the mortgage). 

If you have filed taxes on the property, we look at the net rental income and then subtract out the PITI.

Subtract that payment amount from your current principal and interest payment.