To be honest, wearing the bi-focals glasses upside down is not very effective and wearing them on the end of your nose is not very comfortable. Also, for those locksmith technicians who are getting old (like me), crawling underneath is not really an option, and even if they could looking up far enough to see what they need to see would be very difficult.
My solution was to have the lens duplicated on the top with the same or just a shade weaker, prescriptions lens cut. I can now see through the center of the lens, or if it needs to be magnified by the bi-focal, I can look through either the top or the bottom of my glasses. Check out this link for an honest and reliable locksmith company