How much boiled water to give a 6 week old?

I have a 6 week old daughter, a few weeks back my milk dwindled and eventually dried up so i had to start formula feeding her, this made her very constipated so i took her to my Gp who suggested i give her some cooled boiled water between feeds.. has anyone else had experiences with this and how much water should i be feeding her.. i don't want to give her too much