LTM4653 - optimal thermal PCB layout and thermal theda values

Hello,

1) What is the optimal thermal PCB layout? Does the evaluation board DC2327A represent the optimal thermal PCB layout?

2) The datasheet has several thermal theda values: theda_JCtop = 20.6, theda_JCbottom = 5.1, theda_JB = 6.0, theda_JA = 15.5. Which would I use for analysis of the BGA packaged soldered to the board without a heatsink? I am particularly confused about the difference between theda_JCBottom and theda_JB. I want to take advantage of the theda_JCbottom thermal resistance, but I am unsure what PCB layout and assembly configuration resulted in theda_JCbottom.

Thank you!