The same 12-column grid that works for an A1 poster also works for a 12-inch cover, scaled. Here are three things I've learned from running both ends of the system.

1. The gutters are the rhythm

Gutters aren't negative space. They're the beat.

2. The baseline matters more than the column

A poster without a baseline is just shapes. With one, it's a system.

3. Crop, don't redesign

When the format changes, crop the system. Don't redesign it.