As others have said, they'd need a higher volume of cows milk over powdered. I feed 150%, but that's Jersey milk. 200% is probably a better multiplier for 'ordinary' cows' milk.
If you do feed cows' milk, or half-and-half cows' and powdered, you must use raw unprocessed milk. Not pasteurised, not homogenised, not sterilised.
The instructions on the packets of powdered milk work, so start your calculations there. Some of them are up to 1L per lamb per day, some are 1.5L. Most will suggest you wean at 6 weeks, I prefer 8. And don't wean until they are eating at least 1/2lb cake per head per day, as they must eat cake to replace the nutrients in the milk; at that age they can't get all they need from grass.