You're looking at T+72 but I was referring to T+240...!
I make the T+72 500-1,000 hPa thickness more or less the same as you, 525dam. The formula is the 500 hPa value, in this case 532, less 7/8ths of the SLP difference from 1,000mb, in this case 1,011-1,000=11 x 7/8 = 7, so 525.
(if SLP is sub-1,000 you have to deduct not add)
Originally Posted by: Gandalf The White