For anyone who's interested, here is a link to an article that I wrote on this subject: http://retirehappy.ca/cpp-survivor-benefits/

In a nutshell, the combined benefit is limited the maximum of a retirement benefit on its own, but that maximum amount effectively varies depending on what age the person started receiving their retirement pension. In addition to the restriction of a maximum however, the combined survivor's benefit is always reduced by 40% of the lesser of itself or of the calculated retirement pension.