Got a table here in my Ops Man Gen which says:' when operating within 20nm of terrain exceeding 2000'amsl, commanders are to increase the standard moca/mora by the following amounts'.
Sadly, I don't have the instant technology to reproduce the table, but the figures for terrain fm 2000-8000' seem to fit in with scando's formula. Above 8000', the increment seems to be about 650'/10kts above 30kts windspeed.
These figures are for airline operations though, and not necessarily for light twin/ga.
Rgds to all
mcdhu