Breaking Cable

Our team built a winch with spectra cable to lift our robot this year. I'm wondering how other teams determine what strength cable to use. Here's what we did: I figured that the greatest weight our robot would need to lift is 260 pounds (our own robot plus one trying to pull us down). So we ordered 468-pound test.

To make sure this was adequate, we used the line to lift a student who weighed over 200 pounds. Lifting was no problem, but a little bouncing would break the line. Just to be safe, we decided to go with the 876-pound test cable instead. Since an active competition might involve more than just lifting dead weight, I'm wondering if there's a good way to calculate the correct cable size, rather than just testing it like we did...