First of all, congrats on the new addition!
We started giving DD a FF at night from about 6-7 weeks ish - I think
It really helped her to settle well in the evenings and initially even helped her to sleep for sometimes even 9 hours in a row!
After a while things settled and she got used to the bottle feeds, and she began falling back into her normal sleep routine. (I made up the amount of formula recommended on the side of the Aptamil Box) I think for a 9 wk old its about 5OZ? She very quickly became used to this amount and has since increased to about 6 sometimes 7 OZ at 7pm. (She's 16wks)
Whilst DH was upstairs giving her the FF, I would express off approx 2-2.5 OZ from each boob (though trial and error I think that approx how much she was having from me at the evening feed)
But recently, I discovered that you can actually mix Formula with EBM. So instead of freezing what I express off ( my freezer was stacked out with EBM!!), I now refridgerate it and give DD whatever I expressed off from the previous night, mixed with 3oz of formula. Making approx 7oz.
(I make the formula up about an hour before bedtime and let it cool to room temp, then take the EBM out of the fridge and let it warm up to room temp for 10 mins. I mix the two together at room temp and warm the bottle in a jug of hot water)
She seems to love it and normally has 6oz and will sleep til about midnight ish.
I hope this helps! Good luck