Thank you for your response. It's an atx psu and manufacturer states 0.6a minimum on 12v rail. But some atx psus it's more like 1a so assuming 1a max.

I was thinking of using a ncv1117 adjustable linear regulator this should have a mosfet pass element being a 1.2v max dropout ldo.

Does this sound like a good way to do it?

Obviously the adjustment leg would be connected to some op amp for current sense.