The 50% rule isn't exact but will likely be your average over time. That appreciation is great, it sounds like that investment is performing very well right now and will likely continue to do so.
Some things I don't see listed in your example are vacancy and capex for the big expenditures. Vacancy is more of a reduction an income than an expense. If you can pre-lease it or have a 3 day turnaround this will never come up. You'll also probably need to make a number of small repairs between tenants. There will be other large ticket issues that pop up as well as big things that will eventually need replaced like the roof, furnace, plumbing issues, etc.
If you have enough properties for enough time the below or worse will eventually happen and repeat itself. You will also hopefully have plenty of mild years with almost nothing happening.
One year you may have a $10,350 new roof expense (or other major repair), full month of vacancy ($4,050), and $3,000 of minor repairs to get it rent ready wiping out your $250 per month repair budget. This would leave you at $0 profit. Any other potential expense or longer vacancy would push it negative.
You are fortunate to have not had any large expenses yet and I hope that lasts as long as possible for you.