Where best to massage on the back?

A key question to ask the client with referring leg pain is to ask whether the pain refers over the anterior or posterior of their legs.  If the answer is anterior, the best place to look for the problem is the lumbar plexus and the muscles that run parallel and dorsally to it such as the quadratus lumborum and erector spinae.
If the answer is posterior then kneading the buttocks in particular focusing on the piriformis muscle and gluteus medius often offers good results.  A sharp thumb pressure on the erector spinae muscles just superior to the sacrum is effective too.