I would personally just take a few measured samples of your beer and then add in a premeasured amount of extract into the sample. Based on the size of your sample and the amount of extract you put into the beer, you can figure out how much the entire batch needs. Kinda like wine, it may take a few different samples before you find the right balance between the beer and the fruit flavors.
I used about 3oz for 5 gallons in my last flavored beer and it was quite strong at first. After about 1-2 months of conditioning, it goes down smooth and tastes wonderful. You may notice that for the first few weeks after being bottled, your beer tastes like fruit extract. Just let it sit for a little while longer and those flavors should subside nicely for you.