I set my scopes up on my dining room table which looks out to neighbours house which has white UVPC double glazing and a is brick built,plenty of verticle and horizontal reference points, also they have a white outside light I boresight on 😁,with bolt out I can hold my head and eye down so the verticle line goes plumb down through the centre of bore axis..
I double check with levels,I've boresighted within 4 inches several times