From Formula to Breastmilk or mixing?

Ladies, I am donating some of my freezer stash from when my son was 6 months old to a friend who has 6 month old twins who are currently on formula. 

If they mix the breastmilk and formula, is there a mixing ratio that would work best or does it even matter? If they mix the two, they're thinking of doing 6 ounce bottles.

Also, they were wondering if they give the BM without mixing, would they be able to give a "normal" amount of BM in their bottle, like 3-4 ounces or would they still be hungry due to the fact that they're used to 8 ounce bottles of formula?

TIA for your thoughts and help!