I will be using rental property exclusively myself for a few years. I also do my own maintenance.
Start with the gross rents, and subtract the fixed known expenses. Then, subtract at least 5% for vacancy, 10% for maintenance and 10% for property management. Then, subtract the mortgage payment. I actually then only take ~60% of what is left to even account for more contingencies. Higher taxes, licenses, lower rents, bad tenants, etc. can all kill the profit.
Even if you manage the properties yourself, you should have a contingency for that, use the 10% number.
With Firecalc, use it like a pension. Fidelity's RIP accounts for it better.
In reality, much of the rental income is sheltered by depreciation, so you could actually have two categories, one like a Roth, one like a pension. In my case, I have ~$60K of depreciation, plus many business expenses that help with taxes.
There really is not a good method for rental property, in any planning tool. Even your asset allocation, if you have paid off properties, becomes a bit difficult. I figure my equity is like a bond allocation, so I have a higher percentage of equities in my investment account.
FIRE no later than 7/5/2016
at 56 (done), securing '16 401K match (done), getting '15 401K match (done), LTI Bonus (done), Perf bonus (done), maxing out 401K (done), picking up 1,000 hours to get another year of pension (done), July 1st benefits (vacation day, healthcare) (done), July 4th holiday. 0 days left. (done) OFFICIALLY RETIRED 7/5/2016!!