In the mid-1990s I began using Wilson ProStaff Classic 6.1 racquets. When new they measured RA flex in the low 70s (quite stiff for a players' racquet). Over time the racquets became more flexible, eventually measuring in the 67-68 range before I replaced them. I should add that this was a long time, about 10 years cycling four racquets and playing regular leagues, practice and tournaments. I typically had them restrung every couple of months unless the strings broke earlier. The change in flex happens so gradually you don't notice it until one day you pick up a new version of the same racquet and you suddenly remember the pop and control you had when the racquets were new. My question is - is there a formula of RA percentage change when a racquet should go ahead and be replaced? Say if it starts out as a 70 should it be replaced when it measures a couple points more flexible?