When the fork is fully compressed with the damper rod, spring, top and bottom spring seats and spacer removed and the air bled out of the oil....the oil level is measured from the edge of the upper tube to the 'oil level' in the stanchion(bottom tube)
It is specified to be 118mm +/- 2mm. But there is lots of debate on this subject as regards to springs in or out when measuring oil level and the actual measurement itself.
I've recently done mine on a 02 mille showa fork and it has transformed them.