If I buy a 100K house that appreciates 3% over the long run (per study by famed economist who called the last crash, Robert Shiller), in 30 years I have ($) 242,726.25 Jack - In this example, you are picking a period that includes many market fluctuations - ie a few recessions, etc.

It'll hover there and fluctuate as supply and demand dictate; as well as technical advancements that make shale more cost effective. 

Unless the floor structure is of concern, or the tiles are so large they are prone to breaking if there is any fluctuations in the sub floor.

I do not use the 50% rule for purchasing decisions, but am aware that over time, the annual cost of owning a long-term rental will fluctuate.

This number will fluctuate some from year-to-year as you have more or less vacancy and maintenance, but your initial investment will remain constant.Best of luck with finding that first rental!

Even with water, I've seen some wild fluctuations in bi-monthly bills.

What does fluctuate however, is vacancy rates.  

I don't care if they fluctuate in value. 

The market price of your rental may fluctuate with the cycle but with a long view (see Buy and Hold by Shumacher) the rental market/job market and economy might be more relevant to landlord, because or vacancies and market rents. 

When does 70% fluctuate?