Help with comparing the two lengths

I need help, I don’t even know where to start, so I will be grateful for any help.
I have a picture like this, I would like the length of the red line to be expressed as a percentage, after the hero I created a spline mesh and I would like to compare these two lengths. How best to convert the red line into percentages and then compare it to the spline mesh created by the player.
Thanks for every hint