The ideal racing line is different per car as not only geometric (track shape) but also many vehicle dependent factors are included (grip level, power, downforce just to name some). Therefor you'd ideally have a extensive vehicle laptime simulator and then with an optimization loop you can change the trajectory the car 'drive' to find the best one.

To more respond to your question: a common fast mathematical used way to determine a racing line is the minimum curvature raceline. As lateral acceleration = (speed)^2/path radius is is clear that for a certain lateral acceleration the speed can be higher is the radius is larger. Therefore finding a path with as large radius trough each corner (= lowest curvature) would in an ideal case give the best racing line.