OK, some things are going to be tough to quantify. But I suspect that at least some if not most of your responsibilities consist of putting out fires, right? I am not a sysadmin, but I know ours at my last job ended up fixing lots of stuff. When I was working in an electronic test support group, we used mean time to repair and mean time between failures to track our efforts. it does require logging problems and tracking time, but you should be doing that now anyway.