I think that you were put in a tough situation. I might have just said "Oh, you don't have to give anything! DD will just love that you are able to come," and continue with variations of that if he persists. Obviously, he will still give something, but the amount won't be "set" by you.
It strikes me odd that a grown man cannot figure out for himself how much is appropriate to give.
As for the "cover your plate" rule, I agree with the PPs who wonder how guests are supposed to know how much each plate costs. Weddings can range from a country buffet to a caviar and champagne dinner. And just because a couple chooses the former, doesn't mean they "deserve" any less than the latter. Just give what you can afford, appropriate to the relationship.
edited for typo