I tap it and listen. Then I play music and listen. I keep adjusting until I am happy with the sound. After a while, I learned to correlate the amount of resonance in the plate with what sounds good from the system. I like having the ability to fine tune the sound of the rack to taste. It is subjective.
I find it very interesting that this approach ( tuned high mass ) works for you and David. In the past when I have put equipment on high mass supports (dense wood) it has not worked for me. I have always liked light yet stiff supports.
Recently, however, I have purchased, on recommendation, a commercial Naim Fraim rack which is very different to my past supports, yet has brought improvement.
It shows that there are different ways to climb the mountain.