I'd suggest the following methodology:
1. Go to
http://www.firecalc.com.
2. In the first tab labeled "Start Here" (there are multiple tabs across the top), put $50000 in as your spending.
3. In the second tab "Other Income/Spending", put Pension Income of $22800 starting in 2020. This represents the $1900 a month from your AirBnB. FIREcalc uses annual numbers for inputs on this screen.
4. In the last tab "Investigate", go to "Search for settings that will get a success rate of as close to
100% as possible (usually within 1%) by changing...Spending Level or Starting portfolio value" and click on Starting portfolio value.
5. Optionally, you can change any other inputs to more closely match your situation.
6. Click Submit.
If you don't change anything in step 5, you should see that a portfolio of $756,651 (plus the AirBnB income) is historically 100% safe over a 30 year period.
This exercise does assume several things:
1. That you continue the $1900 income indefinitely and increase it by inflation.
2. That you're including all of your spending in the $50K number, including taxes and healthcare.
3. That the future isn't any worse than the worst of the past.
4. That your situation matches the other inputs in the tool.
Logically this makes sense if you buy the 4% rule. The $22800 you're bringing in represents the equivalent of $22800 / 4% = $570,000 in FIRE stash. The $50,000 - $22,800 = $27,200 you still need is covered by an additional $680,000 from the portfolio ($27,200 / 4% = $680,000).
Also, note that the $756,651 plus $570,000 is pretty close to your $1.25M number.
(The difference between the $756,651 and the $680,000 number is because 4% is not 100% safe over 30 years assuming FIREcalc defaults; it's only about 95% safe. If you change the 100% to 95% in step 4 above, you'll get a number which is very close to the $680,000 number.)