As pointed out to me by KnightMiner, Create has adjusted the ratios of their Brass recipe to be 1 Copper + 1 Zinc.

The old BluePower ratio was 3 Copper + 1 Zinc, and the closest approximation to IRL brass would be 2 Copper + 1 Zinc.

I need to have a think about this, but we should probably adjust to match Create, since they are the only major mod that uses Brass, afaik.