Yeesh....
...where to start.
1. You're over thinking it. You're dealing with Shimano R-500 wheels - if you start messing with them you will most likely shorten their life even more than they usually have.
2. Don't worry about getting a tensiometer. Pluck the spokes all the way around (realize that the rear should have 2 different tones - Drive Side and Non-Drive side) and look for a common tone. Understand that you will never get them all to be the same....why? because rims aren't round. If you have them all the same the wheel won't be true. Just a serving of reality for you. The craft in all of this is understanding where the tradeoff are.
3. Tension listings? Nice - yeah no. Occasionally you can get recommendations from the rim manufacturer, but on prebuilts - you aren't going to see anything in print. Why? because the manufacturers don't want anyone other than themselves or authorized dealers touching them.
A better question would be, "why do you think you need to worry about them?". Shimano R500's....don't worry about them unless they go un-true. Ride them until the croak. If you take the time to figure all this out then rent yourself out to the local shop - they don't really understand wheel building even though they think they do....
