Last time I pointed out that you need to write a program that feeds your model simple inputs and verifies that the output is correct.
Here are some hints on how to do this better:
If you solve your simple test cases with a spreadsheet, make that spreadsheet clean, explanatory and presentable. You are going to refer to it many times in the future, and you will be improving it and adding new cases to it for years.
Store that spreadsheet together with the testing code.
It’s much better if the person who creates (and maintains) the spreadsheet is not the same as the one who writes the code and the automated tests.
Use a testing framework. There is one for almost every programming language—check Wikipedia’s list. I know that you’d rather not learn yet another thing. (In fact, a few posts ago I encouraged you to not learn a version control system, exactly because I know you don’t want to.) However, automated testing is crucial, especially for models. If you don’t use a testing framework, you’ll end up reinventing the wheel (and making it square). What’s more, testing frameworks are easy to begin with, and you don’t need to become an expert.