There is nothing between the plywood and the joist. In my case the DIY
book I have (Ceramic Tile by Creative Homeowner sold at my local Big
Box store) it's clear the reference is for the total thickness of
subfloor and underlayment. Whether or not the book is right is open
for debate, but your jump test seems like a good idea either way.
Again and for the record, I won't be using 3/8, I'll be using 5/16 but
I think your point is the same. Thanks for your thoughts.