Hi Booleano,

re:
> How can I convert the route curve to its equivalent straight line length, in order to do the Flow

You don't need to match exact lengths, instead use the "Stretch" checkbox option in the Flow command, that will match the full length of the base curve onto the full length of the target curve.

But if you still want to make a line to match the length of an existing curve you can get the curve's length by selecting it, then click "Details..." and then Curve properties > Length. Then draw a line and paste in the length to activate distance constraint.

There is also a plugin called UnwrapCurve that you can install that will generate a line of the selected curve's length for you:
https://moi3d.com/forum/index.php?webtag=MOI&msg=5136.1

- Michael