Anyone understand this relationship in the sense whereby, for example, what might be the equivalent string tension in a 105 Square inch racquet to a 90 strung at 45 lbs? I'm sure it's not that cut&dry but there must be some basic guidelines... Thanks

as a really general rule, you can measure the length of the longest strings in each racquet to get a ratio. For example, say your 90's longest mains are 12 inches, and your 105's longest mains are 15 inches. You have a string length ratio of 12/15 = 4/5. Now, your 90 has a reference tension of 45 lbs; so, to get the tension for the 105, you need to find the solution to the equation 45/x = 4/5. To solve the equation you divide 45 by 4/5. You get 56.25 lbs. So, string the 105 at 56.25 lbs and you should end up with the same relative tension (at least on the longest mains). If you want more info, do a search for "proportional stringing" in the forum, or on Grand Slam Stringers.