Sorry for the delay - had to get some help on this one 
Dmitry made a tweek to your document. Check it out at the link below:
https://docs.getgrist.com/doc/igMK7ZEc1ay3~d5Yaz1XSgmKKtYKPkEGZDT~26/p/7
To enter a categorization, select a transaction then select a category in the Target ID dropdown of the empty row in the bottom widget. The amount is auto-populated with the total uncategorized amount but you can change the amount to anything.
Then, the next row will be auto populated with the balance remaining after selecting a category.
Let us know what you think!