I use an ERT300 to get dynamic tension "reference" numbers (DT value) that relate to stringbed stiffness once the strings are in there.
I measure right after a racquet comes off the stringer and then I measure again every two or three times I play. Doing this I got a pretty good idea of what reference tension to set my stringer to in order to achieve a DT that corresponds to a feel I like when I hit the ball. By doing the DT measurements over time as I use a racquet I have a pretty good idea of how long it takes for strings to get too mushy as they degrade and the rate at which various strings degrade. And of course I've been able to figure out which strings hold tension/feel in the comfort zone I like for the longest.
All the numbers in the above are "magic" -- there's no real meaning other than they relate to each other in fixed ratio and using them as a reference I can consistently achieve results that feel good to me.