scott,
The simplest and easiest answer is to take your stringers word for it and change your reference tension two pounds lighter - that way the string jobs you do and the ones he does will be nearly identical.
On another note, if I assume that your stringer is a professional, then I say, "congratulations to you on stringing your own racquet", you've nearly duplicated the efforts of a pro. You are now promoted to the position formerly held by your stringer.
You also ask: " . . . but wondered how I can measure to make sure that my 58 is not 60?" There is an easy answer to the question you WANT answered above, but no easy answer to the question you NEED answered (implied above). The easy answer is to get your hands on a calibrator, and cross reference the reading between the two machines - yours and your (former) stringer. Straightforward.
The implied question is "How do I get my stringbed stiffness the same as anyone elses on any other machine?" Answer: Trial and error, attention to detail, understanding the process, and finally - repetition of the process over and over again to the minutest detail.
Stringing a racquet isn't just about putting a "known" tension on an individual string, it's really about getting a repeatable "string bed stiffness" time after time. And that involves not only the "reference tension" set on the tension head, but it brings in the type of machine: constant pull, pull & brake, electronic, gravity; type of clamps: fixed or floating, single action, double action; type of racquet mounting: 2 point vs 4 point vs 6 point; and the most important variable here is: YOU!
If I can paraphrase the advice given by Chevy Chase's character in "Caddy Shack" to Bill Murray's character: " . . . be one with your stringing machine . . . be the machine . . ."
