Answer: In cold climates, it dies right to the ground and regrows from there. So when the plant is dormant, you can cut it as far back as you like.
If you're in doubt, leave 12" shoots to identify where it is in the garden and to give the plant some basic framework for new growth. It will regrow from there.
Don't prune it in fall, wait until you are sure it is dormant (after a couple of hard freezes) and then prune it down.
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