STR Calculator Troubles! Mortgages???

[David Tardy]

So I am creating a comprehensive spreadsheet that I can easily plug basic numbers into, (Will share once complete) including all STR specific expenses, but I have ran into some trouble when putting an equation together to churn out a reliable P&I number.

From my basic calculations so far I am getting almost half of what an actual mortgage calculator tells me. I take the Price - Down Payment * 1.05 (or whatever the interest rate is) to get my loan amount. Then divide the loan amount by the number of months to pay the loan, so 360. 

Why am I so far off from other calculators?