BelleStar, mixed feeding can work for some, but just as you suggest, milk supply is an issue you need to consider.
At 11 weeks, things have not been going on long enough to be 'iron plated' in terms of supply, esp as you have already intro'd formula, and are not bf at night.
It's the mothers of babies of, say, eight or nine months plus, who are feeding at night and maybe a bit more in the day at weekends, who can get away with fewer feeds.
In your case, 3 sometimes 4 feeds in 24 hours, with long gaps overnight and sometimes in the day, is prob not enough to sustain a reliable milk production. The risk is the body will not just 'downsize' to 3-4 feeds, like it does with older babies, but it dwindles and fades, and the baby objects to it....and you find yourself giving more formula in response, and then you really are saying goodbye to breastmilk.
If you want to sustain bf, then you'd be better off keeping formula bottles to the time you really want them - maybe once a day?
As I say, some people do manage ok, but I would say most don't, and they find themselves stopping bf before they wanted to...and you can't tell in advance which category you are in :)