When you place water bottles in your brewing stand, a comparator will give you a redstone signal with the strength of 4, 8 or 11. But the redstone signal doesn't change when you place an item to brew in the brewing stand.

I know, it will change, if you insert more brewing items (10) than one, but most of the time people brew one recipe not 10 or 64. Many designs have gotten useless after this change and I don't understand, why it was made.

There should be a signal change when brewing, even with one ingredient, so it's possible to use with a circuit. Why not changing it to a layout like this:

Empty brewing stand: 0 (redstone power)
1 bottle: 3
2 bottles: 6
3 bottles: 9
+ 1 to 13 10
+ 13 to 26 11
+ 27 to 39 12
+ 40 to 52 13
+ 53 to 63 14
+ 64 15

or this

Empty brewing stand: 0 (redstone power)
1 bottle: 4
2 bottles: 8
3 bottles: 12
+ 1 to 31 13
+ 32 to 63 14
+ 64 15

At least some kind of solution where you can detect a working brewing stand with 1 item.