My two older kids went straight on to cow's milk, I didn't mix it with formula at all, no problmes.
If you do want to mix it (maybe help with the taste adjustment) mix the formula as per the instructions, and then gradually add more cows milk to that. So make 150ml of formula as you normally would; as you start to add more cows milk, make the formula amount less. Or mix up 150ml & then split it between two bottles.