Ground a dryer converting to a 3-prong cord - manufacturer hardware missing
What you have done is correct but whether it is safe is another thing. If I had to do this at gunpoint, I would have used a Stranded piece of wire with eye connectors at each end. That would give a much more solid connection at both ends without a wire wrapped around a screw over a eye termination. 

To test the connections with a multimeter I'd go from the middle terminal to each end terminal and look for 120V, then from the two end terminals and look for 240V.

You should really do this according to current code and leave the four wire plug and run your existing feed into a new box and add a four prong receptacle.