Replying to Backcountry pole question.

Right so i have a pair of old regular sized poles that i want to turn into back country poles. Normally i ski park, but i have gotten myself a pair of JJs and need a pair of pole that can match that type of skiing. I also have backcountry baskets for the poles, and sup sick handles but i need to know the Length of pole i should be using for Backcountry. When i ride park i ride really short poles like maybe 90cm or so, but have no clue when it comes to backcountry.

The Facts:
Im 5'10
Very experienced skier
Preffer shorter poles.

Sparknotes:

Need to cut down my poles to a suitable backcountry length. Im 5'10 how long should the poles optimally be.

Thanks NS!

Also +K to good answers. -K for hate!