Also, what are the worse renovations that don't add as much value in proportion to the actual cost?

You can cashflow here, but with the rise in prices the rental rates haven't come up proportionally yet.

Since we are retired, when we do need to put in the hours rehabbing a unit, we have the time without the stress of juggling jobs and LLing.My husband and I both have significant savings in 401Ks,  We wanted the keep the same proportion of our portfolio in RE, rather than placing it all in the stock market. 

I doubt you could apply it in that case, the proportion of deck to apt is so lopsided.  

So you need to make sure your base is very secure meaning the slab or if you are blocking them you have compacted material.

If rates are that high, it will be due to very high inflation and your rents and property value will increase as well...maybe not proportionally but directionally. 

There are a few reasons that standout to me for why this might be:1) It seems that the extra rent collected by having a 3rd (and especially 4th) bedroom doesn't always proportionally add to the rent price relative to maybe a stereotypical price hike in buying a 3/2 vs a 2/1 (i.e. maybe a 2/1 rents for $750 and a 3/1 gets $900.

With respect to my own reading:Q1: It seems very clear that "outstanding debt / basis = UBTI" is the proper formula.Q2: I agree with you.Q3: Pub 598 seems fairly clear that depreciation is allowed in proportion to the debt-financed portion (see pg. 18).

Suppose you were tasked to determine a single annual maintenance amount that would be accurate for the two cars illustrated below.One is a new small compact car and the other is an older wrecked Humvee.

And a hint on proportion?