I have an old racket Yonex Iso-77VF which needs to be restringed as the string is too relax now. But my question is - what is the tension I can do? Suggested on the racket, the Yonex says the tension is 18-20 pounds however does it seem to be not good enough? Can I do over 24+ pounds as my other rackets usually are at least this level ? Does that sound silly? Am I going to take risky? Any ideas? Thanks.