Thanx for them replies everyone!

Learned some new techniques :)

But even if I do find the exact point how do I pick that exact point from where to start moving the curve, as I can see it transform tool only moves whole objects not portions of the curve so I still have to handpick the point even if I can get it more precisely now thanks to you guys.

Lets say the guideline box is 156mm wide and the curve with it's mirror side combined needs to be that width after snapping the curve to it, I am still getting a width of 155,876mm for example. I can live with that but would be nice to get the exact width if possible.