[Fot] Spitfire Pinion height question

[don at dcphotos.com]

All, 
I am rebuilding a 4:11 diff from a Spitfire.

The factory manual (the three ring looseleaf one) says:
“Distance from head bearing abutment face on pinion to centre of crown wheel bearings. 3.000” “

Can someone decipher that for me? 

I’ve measured 3” from pretty much everything I can in there and can’t seem to correlate it to anything.
Thanks, 

Don