Quick test on my normal PC monitor, holding gun above and pointing down, up to ~30 degrees seemed to work as well as straight on (90 degrees being straight on). Horizontal was great, but vertical position was harder to hit, which could have been a combo of the gun not lined up with my eyes, math fact that same pointing error is magnified at the sharper angle, and/or maybe the lightgun being less accurate. I think it was almost all the first one and partially the second. BTW, I calibrated the gun straight on; tested during bright day light w/ white curtain west facing window behind me @ ~4:00pm local time (ie: lots of background light).
Edit: Added testing conditions.