selecting weight for shock

[ParanoidAndroid]

So today I finally got some balls and attempted to do shock. I'm not sure how well it went though, my joints felt the shock more than anything else. On a few exercises I felt that I went to light, and on others I think I went to high. I was trying to go about 10-15 pounds heavier than I would for doing about 4 reps. I'm just wondering if there's any sort of guideline for estimating how much weight to use when doing 1-2 reps.
Thanks