Done to a turn is an old expression that first referred to food, especially meat, cooked exactly right. Its origin comes from spit roasting. A piece of meat was turned over a fire, and if the turning was well judged, the meat was evenly cooked and not burnt.
That physical turning explains the phrase. Something was done to a turn when it reached just the right point. In modern English, the most literal use is still about cooking, but the expression can also describe anything carried out with exactness or excellent timing.
- The lamb was done to a turn, tender inside and crisp outside.
- Her short reply was done to a turn, polite, clear, and perfectly timed.
The phrase is now a little old fashioned, so you are more likely to see it in literary writing, historical fiction, or careful journalistic prose than in everyday conversation. Even so, the image behind it is easy to understand, which helps explain why the meaning expanded from cooking to anything done just right.