They're pretty good at holding tension, but I would use 4 clamps for mains and two for crosses at those tensions. It doesn't all have to be Yonex quality clamps though. I find that it's better to clamps the mains in a staggered formation so say you wanted to clamp the 4th main string. First clamp goes on 4th and 3rd string. At the same time clamp the 3rd and 2nd string right up against your first clamp. I'll show you a picture if you want to see how it looks like. I find it better than putting both clamps on 3rd and 4th strings.