There is only 1 place where soldermask clearance should be defined and it is in the footprint.
This idea of global value for this and KLC forbids to put a value in footprint is just plain wrong.
It may seem like a nice idea, and hierarchical and logical and nice and what not.
But it's just wrong.
The pcb is made up of footprints and traces.
Footprints make up the pcb, not the other way around.
The pcb itself should not have a saying about footprints.
Pcb manufacturers always process your layers anyway, so you don't need to worry about advancing or different manufacturers capabilities.
It's just wrong, and KLC enforce this bad thinking.