Anyways, I don't want to be the one that causes you to under-size, but I think you'll likely to be OK with a 60k Btu/hr unit. 

I am investing in a medium-sized apartment complex right here in the city.

Great size, great diverse economy and strong rental market.

There were obviously people who made a lot more, but I am trying to guess on the lower end, probably anywhere from 23k-26k (there were people with similar sized properties self managing at 40-50k gross). 

You can also have an additional attached ADU, but need to comply with size rules (up to 25% of main house) and are supposed to get permits and approvals. 

There is no "small size" exemption. 

You want to take in account size(sqftage), number of bedrooms, features (garage, pool, etc).

Some people mention calculating the percentage based off of rented bedrooms vs personal-use bedrooms (1 personal use vs 2 rented and doesn't take into account square footage size differences), others talk about using the personal use square footage divided by the total home square footage (includes common areas) and others mention that the shared spaces do not count towards a Schedule E.All in all, I just want to make sure I'm calculating that percentage fairly (both to me and the government). 

I've lived in San Francisco and Chicago, but prefer the medium-sized midwestern cities.

The lot size for for an ADU build.