What has worked for me is, lay a metal ruler along the cut, if possible clamp the whole thing, then with a blade start marking along using the ruler as support for the blade. But the trick is to try to make a "V" cut, by tilting the blade to one side and then to the other on each pass. After you have deepened the cut to about half the veroīs depth, just snap it with a swift movement. If the cut is not deep enough it may snap outside the mark.
Careful with the blades. Always make the "strokes" away from you, never towards you. Donīt use fingers to hold the ruler/vero, although Iīve been guilty of doing that.
I eventually got a Dremel.