How many wheatgrass seeds per tray?

Hi,

As I'm starting to get serious with wheatgrass now, I need to get everything exactly right! For tray sizes of 38cm x 24cm x 5cm (like the top one listed here http://www.thegardensuperstore.co.uk/acatalog/Seed_Trays.html) how many seeds would I need per tray for that size? I've heard anywhere from 2.5 cups of seeds to 500g, but I don't know what size trays they were using anyway! I'd like to know in advance as I'm going to order wheatgrass seeds in bulk and calulate everything out so I have a long supply.