Unfortunately, the instructions in the test kit don't tell you how to use the base demand test. The acid demand test is pretty good, though.
So you plug in what you see. Is the color close to 6.8? Then plug that in for now. You might be lower than that. But you have no idea how much lower. So you have to work up to it.
As I said in our lengthy PM exchange, plug in your pH and TA values into pool math. Use 20,000 as the volume. See how much Borax it says to add to adjust pH, and add half that much. Let it mix half an hour or more then retest pH and TA. You need both to do the calculations you need. Then add half the Baking Soda it calls for to get to 80 TA and let that mix. Retest pH and TA again and just plug in what you see, even if it's still below the lowest color. Then add the Borax it calls for. Let it mix and retest pH and TA again. When the TA gets to 60 or above, concentrate on adjusting pH only using Borax. At some point, when you enter 6.8 as now and 7.4 as target, you'll see the color shift. Then you can do this
At that point, you're in the normal zone and things should get really easy.
Once you start doing it, it will make more sense.