Now that I re-re-read that page and the post which directed me there (curiously, one of yours, Tut (1646/16)) I think it’s just a wording issue in the guide. My interpretation here also seems to have affected how you just interpreted it, too, as you actually explained it clearly in that linked post from 2018 
. What I now understand it’s saying is that it isn’t added after the subtotal like a regular tax component is. Thanks for helping clear that up!
Everything else you’ve just explained also makes sense, and is how I suspected it works, cheers.
I’ll have to weight up making sure I have a proper worksheet set up, or just going the other route suggested by lubos somewhere, which is manually translating the line items to be tax inclusive upon entry (or using something like (lineTotal / numUnits * 1.1) in the unit price).