Question about cash flow calculation

[Vasudev Kirs]

Hello:

I am not sure what interest rate to use while calculating cash flow. Is there a standard formula that takes into account the fluctuations of interest rates?

Here are the details at a high level:

Purchase price: 174,000
Closing costs: 2,000
Down payment: 20%

Rental income: 1525
Monthly expenses: 487 (insurance + tax)
Maintenance: 128 (1% of purchase price)
10% vacancy: 150
Self-managing

I am not sure what interest rate to use to get the monthly mortgage amount. I have excellent credit score (above 770 and no other mortgages). Can someone please help me?

Thanks!