If fare buckets weren't limited to 9, the numbers on a truly empty flight could be something like:

J112 C92 D72 R52 I22
Y148 B128 H108 .... O9 G9

This is on the basis that they could position the curtain anywhere or nowhere in the cabin, and ignoring the catering issues for now.

(It's unrealistic but there does come a point where you can only sell Y by reducing the number of business class tickets available: bear in mind that each CE seat comes with 1/2 an empty seat).

In the example above, every seat sold would have to reduce the numbers of J available to sell. If people are paying similar amounts for a Y seat over a C seat - or even 2/3 the price if you're going to sell them all - that's a good reason to move the curtain forward; if that looks unlikely to happen then you can adjust the revenue mix by throwing more "cheap" CE seats.

However, while reading fare buckets is an art and not a science (akin to tea leaves really), and not something I'd claim to be particularly skilled at, I'd suggest that it makes no sense to consider the numbers in any cabin without considering the pressure from the cabin behind it, whether the result is that passengers or curtains move forward.