My DS went for 7 to 10 days between poos from about 3 weeks til about 14 weeks. Even now at 17 weeks he only poos twice a week. Think of what you're saving in nappies and wipes! Beware though that the farts get more and more smelly the longer they go between poos...
In that case, double and treble check that you are using the correct amount of powder in her bottles. It is very easy to put in too much powder for the amount of water you are using and it can make them constipated.
If your baby seems happy, I'd still leave it another day or so before talking with your HV.
Even ff fed babies can go a couple of days between poos, not sure from how young though. DD certainly did. Does she seem OK in herself? Are you still seeing the midwife? If so, ask her what she thinks.
I'd give the HV a call and maybe get someone to see her. If she seems fine then do nothing for now, if she seems uncomfortable (they pull their knees up and scream if their tummy hurts) try an oz of cooled boiled water in a bottle. I know my HV suggested a teaspoonfull of Apple juice in a cup of water when DS got constipated but that was when we started solids so he was quite a bit older than your DD