I hemmed mine to about an inch above the bottom of the fell on my kilt. In other words, I put on my kilt; put on the jacket; marked where the fell is in relation to the bottom of the jacket; then marked the hem one inch higher than that.

I folded the jacket along the hem line and pinned it to get an idea if that was where i wanted it. I cut the jacket a couple of inches below that hem line, so that there would be fabric to turn up, and it wasn't just a raw edge. Hope that makes sense.