Hi lgsf,
1mg clonazepam is roughly equivalent to 20mg Valium according to Dr. Ashton for the purposes of tapering.
It's suggested that a crossover is done in a number of steps, usually at least four, for instance exchanging 0.25mg clonazepam for 5mg Valium every week or so. Valium is longer acting so will therefore take longer to build up in his system than clonazepam will take to leave his system. There are some more formal plans in the Ashton Manual.
http://www.benzo.org.uk/manual/bzsched.htmhttp://www.benzobuddies.org/benzodiazepine-withdrawal-methods#substitution-taperLiquid titration is another option for your husband. It entails making minute daily cuts that allow for a smoother more comfortable taper.
http://www.benzobuddies.org/forum/index.php?topic=6393.0