I find the @Computer context a tricky one as well. With documents in the cloud and a growing number of connected devices, it seems that I seldom have less than two options available for completing many computer-related next actions. I would recommend the recent Getting Creative with Lists webinar....there is a great piece in there about choosing activity-specific contexts that I think applies really well to the @Computer list.
I try to actually visualize myself performing the action to help me choose the context accordingly. For example -- while I could browse that cool new photographer's website anywhere (iphone, ipad, work PC, home Mac), I picture myself doing it at my home Mac....so @Computer-Mac is where it goes. I could update my travel expenses spreadsheet almost anywhere, but I picture myself doing that at the office (@Office for this one).
At the end of the day, though, I think Oogie's advice is great....pick the first context that comes to mind and put it there. You are only stuck with your choice until the next Weekly Review anyway. You can move it then if you want to.