avoir le compas dans l’œil

To judge distances and proportions accurately, just by eye

It describes someone with a trained or natural knack for measuring things accurately without a ruler or any instrument, an artisan's eye, essentially.

Le menuisier n'a même pas pris de mesures, il a le compas dans l'œil.

The carpenter didn't even take any measurements, he's got an eye for it.

First recorded in the 18th century, in Saint-Simon's memoirs describing Louis XIV. The phrase is also often linked to a remark attributed to Michelangelo, about judging proportions correctly without needing to measure them.