Oh hun I am really sorry that Angus is back in hospital. I hope he is better soon and back home where he belongs.

As for expressing, 100-120ml is excellent!

I think you have done a good thing though, the formula will keep him full so he will sleep longer which means you can get some sleep which will help with the milk.
I switched to formula top-ups when Brendan was feeding every 1-2 hours, I was so exhausted I wasn't making anywhere near enough for him.... and I was expressing for him. He lost some weight and I was told that if he didn't have a gain within a few days they'd re-admit him to hospital. It was the best decision for us and I have never regretted it. He gained so well on the formula