My trick (which usually over estimates by about 50%, but I take this into account) is to look at a portion of veg I know feeds two (or one, or whatever). E.g. I know the size of a head of brocoli that feeds two, I know one carrot is a good portion of carrot for one etc. Then I estimate from that to work out how many portions of veg I have in hand.

Then, usually, I add them all together on the 'more-veg-must-be-good' principle that you mentioned above. But I'm sure if I could follow through this would work ;-)