I'm in a very similar situation!!
My son is 15 weeks and I started EASY including DF last week. I'm also bfing and DF is expressed milk in a bottle - although I only get 4 oz (120ml?) each time, so that's how much I'm giving him. Sometimes I don't get enough milk, so I give him 4oz of formula instead for DF. This doesn't seem to change the time he wakes up to feed, FYI...
I do DF at 10 to 10:30pm. The first night DF was breast milk, and he woke up at 5:30 to feed. The next night with formula, 4:30. The next night with breast milk, 4:30 again. Last night with formula, 4:00. It's creeping up for some reason. The bedtime is between 7 and 7:30, depending on when he falls asleep, but the last feed is always around 6.
He also woke up at DF two nights ago and almost did last night as well as three nights ago. I posted about this, but was told that basically it's up to me whether to continue DF or not...
I was also wondering if I was not giving him enough at DF. During the day, I'm trying to move to 4-hour routine, but in the afternoon it's more like 3 hours. He is eating 5 times (7, 11, 2, 4:30 and 6).
I hope you can get some answers here, it would help me also.