Errr... with the milk replacers I am familiar with, 200g of powder makes 1L of milk... a lamb needs 1L / day for 6 weeks, that's 6 x 7 x .2 = 8.4kg per lamb. Plus they must already be eating at least 1/2 lb creep per day and either grass or hay or both.
When the milk is stopped they will eat more grass and drink more water, which could indeed make them mucky about the backend.