Technically you measure down the middle of the bend. If the vent uses the wall of the cabinet be sure to do the end correction too.
Or ignore both, they will likely cancel each other out or you will get a box tuned 1-2hz higher or lower than you planned. minor.
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Paul Niwranski
1966 Cadillac Superior Crown Royale Limousine Hearse

If you're mirrors ain't shakin...you got taken...
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