This is from the BP rental calculator and is exactly what I'm wanting to see, just with extra payments each month. 

If a client complains, you can always make a quick call to boost it for the month and then revert back, but I have never had to do that.

These platforms typically have billing policies where you pay a per unit fee per month to the software and you get regular updates and support each month.There are a number of low-cost platforms available that are able to offer you a lower price by passing along some of the costs to your tenants (payment processing fees, applications, tenant screening, etc).

I have a couple STR investors do this here in the winter months depending on the events coming to town

This in a neighborhood that is slowly appreciating, some renovated homes mixed in with older homes, but is street by street:Purchase price: $130,00001% rule: meets about 0.88% Down payment 20% and closing costs: $29,493Rate: 6.99% conventional 30 yearMonthly payment PITI: $1009 (was $859 until property taxes increased recently) Current rent $1200 (increased from $1150)Property management fee 10%: $120 (was $115) Net cash flow (if no repairs called in): $71 (was $176)Net rental income most months: $170 (first month rent), $935, $922, $1035 (full month rent minus PM fee) $567, $760, $307, $963, $1035 (full month ), $535, $925, $1035 (full month but now property taxes increased), $585.

I am in PA and yes, it is legal to charge late fees to your tenant regardless of Section 8 subsidies or not, provided the tenant's lease allows for them.

I believe I can do this with the upstairs rented out at $1,650 a month and $1,750 for the main floor unit. 3bed 2bath down and 3bed 1bath up.

Or a seller finance calculator showing the monthly payment based on negotiated price, interest rate, down payment, etc.

Also, how does one assess whether or not one can turn a profit after paying principal/interest, insurance, and taxes each month?

Even though your PMA expires in two months, you may be able to terminate earlier if the agreement allows.Follow the Terms: Send a written notice of termination as specified in the PMA.