Wait until danger of frost has passed. Where to chop depends on what you want. The higher the chop, the more immature the final image will be--this isn't a negative, just a fact. younger trees are thinner as they grow up. Lower chops will force more dramatic taper into the trunk as a new apex is grown out and make the final image more mature or even ancient. The final tree will also be a lot smaller with a lower trunk.
Your trunk now is pretty uninteresting and uniform diameter for more than a few feet. I think you'd want to force a lot of taper into a shorter tree with it and skip trying to make that telephone pole trunk into something more by keeping it tall. If it were mine, I'd chop at six inches or even lower and aim for a final image with dramatic taper and a final height in the ten-12 inch range.