It would be quite tricky to figure out the % based on the wall thickness, I’m no mathematician!
I use Chitubox for my slicer and it has a % based hollow feature which works quite well. It could also be that because the OpenLOCK clip is quite small, that with 2mm wall thickness there’s no space to hollow it. Try loading in a piece with more thickness and see what it says.
As resin printing is becoming more popular more of our pieces we’re providing pre-hollowed and pre-supported as they come out.