...The best written instructions on the topic I have seen are that the plates tones should be even all over the plate and that the back plate should be somewhat higher in frequency (tap tone) than the top plate (belly).
...The acceptable range is at least 1/2 tone (one half of a full tone difference) and no more than a full tone difference. Anything more or less and the instrument will never be successful (no matter how long you wait!)
...After 15 years of learning, building, testing and research... I believe that this the following is good solid proved-by-experience and experimentation advice:
...The top plate should have a Tap Tone of 176 HZ, evenly, all over;the vibrating surfaces.
..The back plate should have a Tap Tone of 198 HZ, evenly, all over;
the vibrating surfaces.
Importance of tone separation of plates...