AusBonsai

I collected this Celtis a year and a half ago from a swampy park near my house. I've just transplanted it from its original growing box to this smaller training pot, removing what was left of the tap root in the process. I think it needs to be chopped some where along the ugly straight section of trunk in the middle, using one of the existing branches as a new leader. Where would you chop it?

Trunk width at base- 50 mm
Total height - 440 mm

Any advise will be greatly appreciated,
Cheers
Sam