Your payment will be ~$2600 to $2700 per month not including taxes and interest.

Most work like this: Rates: 9% to 13% (Most Deals are 11-12%)Terms: up to 36 Months (Most Deals are 6-12 months)Fees: 2-4 points(%) of loan amount paid at closing (Most Deals are 3 points(%))Minimum Loan Amount: $50,000 (For loans less than $250,000 we charge $2,500 minimum)Max Loan: 65%-70% of After Repair Value(ARV) 100% Rehab Financing Available (Require 20% of purchase price down payment or cross-collateral)Closing Timeframe: 48 Hours - 3 Weeks (Most Deals are 2 WeeksWe have private lending partners throughout the entire United States that cover the following states: AL, AZ, AR, CA, CO, CT, DE, FL, GA, HI, IL, IN, IA, KS, KY, LA, ME, MD, MA, MI, MN, MS, MO, MT, NE, NV, NH, NJ, NM, NY, NC, OH, OK, OR, PA, RI, TN, TX, UT, VA, WA, WV, WI, WY.

Important nuance here is that the DSCR Lender will qualify the rent based on the income for the property (i.e. the DSCR Metric) but the value will still be based on residential methodology (sales comps method, not income capitalization or cap rate approach)

Does anyone have any set methods or systems in place for calculating these percentages? 

The 4% interest rate is an important part of this calculation because the principle paydown portion of your payment is significant, I'm going to guess it's about $9k per year.

 $300k/1945 is 154 months (12.85 years) to have the cash flow recover the additional down payment.  

Likely due to the recent hurricane Michael.we have included coverage for replacement of lost rents for at least 6 months, to cover us for the mortgage payments if a hurricane destroys the property and we have to rebuild.  

There are a lot of local lenders that charge a 1% origination fee and the interest only payments during construction are prime plus .75 - 1.0%.

That said you could ask for a statement showing their recent payments.

I provided an up front $10000 earnest money payment and then secured a monthly interest only loan with principal being paid upon final sale.