And since it's not a firm amount for a set number of payments, it won't be coded like a mortgage, but often it's coded like a credit card. 

That's what I also came to the conclusion too after reading through further IRS code.

all my properties are in the 61104 area code 

There are a ton of calculators for excel if you go to the top tool bar and select tools, and then FilePlace: Free Forms and Files.  

You can also click on 'Browse' on the right of the search bar and you will see a big list of demographics, behaviors, etc.To select the type of housing you want to target you would go to 'demographics' and there you can select the type of home you would like to target by size, value, type of residence, etc.Let me know, if you have any questions, mate!

It is very important that you calculate right metrics in the right way..else you can get stuck with a wrong deal....Zip code is a too large of an area from RE perspective.

You can filter the data by zip codes you want to buy in and when the homeowner purchased their home.

I'm not familiar with the codes in your city/ town.

We have a house we love in walking distance to a cool bar and restaurant area (Westport for those familiar with KC).  

@Eugene Kemp I did check the Texas Property Code on this topic and there doesn't seem to be anything stating a particular limit on the late fee you can charge.