Thanks everyone. I have been lucky with my measurements on this project. Before beginning I bought a new tape measure. Normally I buy the cheap ones, but this time I got a high quality tape and it is a real pleasure to use. The lock actually works very well and the case is nicely squared at the back to give accurate reading when you take inside measurements and have to include the length of the tape case. It was well worth the high price.
Mike, an American living in Norway