Using Path class: move object to 50%?

Yesterday I asked how to find the midpoint of a curved line and I was pointed to the excellent Path class:

But I’m still stumped. I can trace the current location of an object to see that it’s x percent of the way through the line, but I want to calculate in the other direction. I want to be able to say “put this object at 50% and put this other object at 75%”.

I’m sure this is possible, but I’m not understanding how in the docs. Anyone have experience with this?

Thanks,
Jon