Just checked Rip's book and its 8.5 inches - based on a standard plate size ( olympic 2" I presume).

My 10kg (standard 1") plate only gives me 4.5 inch so I need a 4 inch block to get me to standard height

I tried other weight plates but the 2.5 & 5 kg ones I have are obviously smaller and are just two slippy. I also now realise the height I was using today was a little too high as I aimed at mid shin and 8.5 inches is below my mid shin