Well as I have learned it flaked barley is used to aid in head retention and percieved mouthfeel of the beer. Most recipes I have seen seem to haver around 10% flaked of the total grain bill. So to get the desired effect from the flaked barley a pound maybe a pound and a half should be plenty. Then just increase your Marris Otter to make up the difference. Otherwise just go with what you got and see how it turns out
. Still will be good beer.