Hi there,
I'm not sure if I'm reading this right, are you doing the DF between 10 & 11pm? Or three hours after that?
If the first timing then it is the right timeframe as recommended by Tracy. My DD always took a full feed at this time, but I think the key thing is does it affect his sleep, or extend it further than if you didn't do the DF. If he is still waking at the same time for his next feed it may not be worth continuing, but it's hard to tell if you've only be doing it for a short while. Sometimes it takes a while for them to get used to it.
How long have you been offering a DF?