Precision of snapping to vertex How could I pick which vertex in scaled object should be snapped onto vertex?

I can only snap the other vertex of the arch to the wall vertex

I would like to snap like that(i can only achieve this by eye without snapping to vertex) - Yellow circle is the state i would like to achieve with snapping to vertex


Ps.
Selection to grid dont work in manual case(screen 2) because it modifies top segments of the arch as well because I would like to keep proportions of the arch segments.
A simple workaround: extrude two vertices on right X. then spin 180 degrees around the center of top face, with 16 steps, then remove doubles (two vertices will be overlapping but not jointed).