I left a few centimeters above the next apex bud for it to die back a little. Its probobly going to have this procedure performed a few more times in its life so no need for surgical precision at this time.
This one is going to go into an akadama / fuji grit mix. It was in a 3 part mix of sharp sand , john innes and horticultural grit - which worked extremely well and may have contributed to its explosive growth. I think i will use that soil mix again in the near future.
The work on oaks will continue with planting of a few more acorns. I got lucky and picked up a few acorns for quercus brantii while walking in a restaurant garden - so il see if i can get them to germinate as well.