You multiply miles by mileage amount and that gives you a tax deduction.

Take that time and multiply it by the attorney's hourly rate, then discount it by their "efficiency." 

Now multiply this error by the entire house, by 4 units, by 8 units.

Still, if you look around at the colleges and universities the cost per credit hour multiplied by the hours for which one might enroll for a single year of college can easily exceed even the sum that I paid.

Looking at Cap Rates, Gross Rental Multiplier among other tools. 

So you multiply by 75% instead of 50%, and put taxes on the back end of the equation (taxes aren't explicitly in the 50% rule, so we need to add them somewhere).

From your example, I divided 7%(.07) by 12, and then multiplied that by $25,000 to get the monthly payments of $145.83.

Multiply the principal by the interest rate then divide by 12. $100,000 loan x 11% interest = $11,000 / 12 = $916.67 monthly payment

Then multiply the square footage by the going rate for your area and see whether you'd come out ahead or behind.What would you do with the space?

Now take that $400 and multiply it by the above 20% tax rate and you only save an incremental $80 on your tax liability over the standard.