Is there a specific string stiffness number that is considered as a line - For eg, if the stiffness of the string is above that number it is considered a stiff string or if the stiffness is below that number, would that be a soft string? I have access to the USRSA and got some numbers. Natural gut usually has a stiffness in the low 100s like 110-120 where as some synthetics like PSGD has 210. So, would PSGD be considered a stiff string? Should we also factor in which racket we put the strings on? FYI - I am using a K90 and would it cause arm problems if I string it with PSGD. Let me know your thoughts.