Start measuring at the middle node (where the two lines intersect), let the measure path snap to one outer end node, keep mouse pressed, tab 'Shift', now drag (with mouse button still pressed) to the other end node until snapping occurs.
Start measuring at the middle node (where the two lines intersect), let the measure path snap to one outer end node, keep mouse pressed, tab 'Shift', now drag (with mouse button still pressed) to the other end node until snapping occurs.