Ok, so of course the minute I ask a question on the forum, I then find the answer on my own. Oh well. Thanks for reading at least and hopefully this post can serve as a useful resource for someone else.
If you want a cell to show the difference between L17 and L16 as mentioned in the previous post, in the cell, enter the equation "=L17-L16" (without the quotes) and that's it. Seems pretty self explanatory, but I had a hard time finding it for myself. This website proved useful: