I guess it depends on how long a bare trunk you want. For myself I'd probably go for a crown high enough to walk underneath (so 6 feet or so of clear trunk) - that's how I'm training a couple of young sorbus that I bought as very young whips. The advice in my RHS pruning book is each year prune the side shoots of the bottom third and half the length of the shoots on the middle third, until the clear trunk is the height wanted.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
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