Q: How to plan for retirement with rentals / comparable 'rule' to the 4% rule

[Robert Brown]

it's something I've been looking for for a while,  and it doesn't seem to be talked about much (at least where i'm looking)
- the recent BP: Money ep #377 with " Mr. Money Mustache"  prompted me to write

For my specific scenario:
I'm "House Rich" ( or house  poor.. i guess) and the plan is to continue buying more RE as investment rentals vs investing otherwise, for now.
I Have 7* doors, and am actively looking for another duplex/tri.
(* i live in one unit, free. and profit from the remaining 6.)
currently my net rent is  more than I budget to live off of,  but not with  any sort of excess , yet,  to  allow me to retire.  Perhaps at 10 doors I can slow down a bit work-wise

The 4% rule of course would allow a i.e. $40K yearly draw of $1M in investments, but there is a lot of stability build in with an index fund vs rentals/

I'm looking for "rules of thumb"/resources/thoughts.. on how many  doors  I may need to retire.. and yes i know that  there isn't a one-size-fits-all answer, but  other's guidance would be helpful.

in S.E. Wisconsin, I'm Netting ( before repairs/vacancy/CapEx) ~ $20K/yr with 6 rented doors. with about $100K spent ( down payments) or arguably less, as i do roll over my profits into new down payments.


Outside of simply calculating/assuming what I'd need to retire, then ensuring that my ( actual, bottom-line) NOI covers that. What are other factors i need to consider?

- do rent increases  generally outpace inflation?
- do property values  generally outpace inflation?

as the properties are paid off,  I could of course sell/refi in my later years if required.

What items am i missing to think about?

Much thanks!
Robert