Firstly, roundabouts aren’t perfectly round.

The city I live in is relatively small but has dozens of roundabouts. They might look round as you drive through them, but if you look from a satellite image most are just “mostly” round, and some that seem round on the ground are plain oblong.

With that in mind, you’re dealing with curves, not a perfect circle.

Different points are defined with a range of methods like survey, measuring, and just plain eyeballing.