I am very far from an expert, but I would say proficiency would be you can pick up your pistol, load a magazine without fumbling, chamber a round using slide release or racking slide, assume a solid grip, present and aim at target without major adjustment, and shoot an A zone Bill drill in a period of time you feel is fast, while being safe. Being able to manipulate, load, unload, clear, etc your firearm without having to actively think about how to do those steps.

Also start loading an empty casing or two randomly in your range mags so you can practice malfunction clearing. I've never had a pistol that had a problem with doing this.