How do I setup the start point of the toolpath?

For the Profile Toolpaths you can use node editing to place the vector's Start Point where you want it.

In Aspire and VCarve Pro, be sure to check the box that says Use vector start points (don't optimize) when setting up the Profile Toolpath as indicated in the image below.

In Cut2D, the default action is to use the vector start points under most conditions and the user has no external control over it.

eg. Start point in Aspire/VCarve Pro

vector-start-point