Every site I've looked at gives the EBC as 1300 - 1500. Have you tried upping the EBC to see what it does to the percentage? Maybe to 1400?
Having actually now checked the pack I've adjusted the EBC to 1400.
I doesn't make a great deal of difference - 460g instead of 500g to achieve the same colour.
Had a little chew on a pinch of the CS3 and it has a surprisingly mild taste though.
Funny thing is if I'd had bought de-husked roast barley again instead of CS3 I'd have just chucked the 500g in without giving it a thought, as I've used that much before in a weaker stout - and it was good.