Weaning bf to bottle - worried

[Whickc]

My dd is now 20 weeks and I am slowly weaning frim bf to formula. She now has been taking a bottle at about 6.30pm and sleeping through to 10.30pm when we wake her for another bottle feed. My concern is that she mostly only takes 4 ounces of formula sometimes 5 at each feed and I'm worried she may not be getting enough to eat. She is (touch wood!) sleeping really well and most nights goes through to about 7am when I bf. The next feed I am trying to switch is the 10am ish and I tried today but she only took about 3.5 ounces. DD is small, 5lb 8 at birth and about 121b 7 now. HV has said not to worry as I am still bf partially so dd will make up extra from bf. But I worry that as I drop each feed she will eat less and less?? I'd be very grateful for any guidance. DD is