Specific mobility

[TallDad71]

Trevor
I have spent some time measuring the specific mobility of some of my builds they fall anywhere between 15 and 18. Your book suggests that 18 is a good target value, I wonder if you could elaborate as to why?

My latest build has a specific mobility of 19.42. However the X braces at the small peaks are lower than usual, the result being some bowing in the soundboard under full tension. My first thought is that more than 18 is perhaps synonymous with braces that are insufficient.