(by John Part)

The goal is to checkout all scores from 61 to 100 witha a maximum of 6 darts.

Checking out with 2 of 6 darts earns 3 points, finishing with 3 darts grants 2 points and a 4 to 6 darts checkout rewards 1 point.
The exception here is 99 where a 3 darts checkout rewards 3 points since it cannot be done with less. 

The darts 4 to 6 are thrown at the remaining value, e.g. your finish is 90 and you get 50 points with your first three darts, you're left with 40 points
to checkout with darts 4 to 6.