Basically, as in your image, extrude the vertex in Y direction of a given distance. In pic A, I extruded 16.8 there with Edge Length turned “on”. In pic B, I used that extruded vertex as a point to “snap”, to knife cut vertical; [K] for knife hot key, [Ctrl] + LMS the vertex, drag the cut line up than MMS snap to vertical, LMS to end cut layout process, then [Enter]. That’s the way I do it with Blender 2.49.