Without a Pro membership you should be able to get 5 free calculations.
Also, search webinars with Brandon Turner. He gives demonstrations on using the rental calculator. It is very important to not forget capex, maintenance, vacancies, and management expenses (even if you'll be your own manager, you may not be forever). I'm no expert like many on here, but, capex can be tricky in my opinion. For example, my duplex has brand new HVAC's on both sides, and a roof thats 5 years old. So capex isn't a huge worry for me. But I still set aside 5% of gross rents per month in a reserve fund for future capex. (Full Disclosure here; I don't actually spend any of my net income, so I actually set aside 100% of my net income for future expenses, retirement, or another rental property purchase). And I'm only 4 years from early retirement, so real estate will not be replacing my full time W2 job.