From Papazian's HBC:
5# amber malt extract
= 5# light malt extract + 2 cups crystal
= 5# light malt extract + 1/2 cup black, chocolate or chocolate malt
FWIW, I used his calculations to make a robust porter- haven't tried it yet but next weekend should be bottling and can let you know.
PM me if you want me to let you know next week.
Manager & Head Brewer
Swan Lane Brewery