That’s pretty much it 
First you plug in what you have in seed_str.
Then you change the 0 in getPossibleWord(word_list, seed_str, 0) to the position of the word you are not sure about, from 0 - 11. So in @ScotterMonkey’s case, 3rd word, so that would be 2
When you run the script you will get all the words that could fit in that gap and still pass the checksum.