Breast Milk vs Formula

We are slowly transitioning over to formula (I have been exclusively pumping for the past 6ish months and my supply is starting to deplete and I’m TIRED). We have to do soy formula because he has a milk allergy. Is it normal for him to be eating about 2 oz less per feeding? Normally he eats 6 oz at a time, and 30-36 oz a day. With the formula mixed (half and half) he’s eating about 4 oz at a time and 20-24 oz a day (sometimes a little more). I’m worried he’s not getting enough!