I just bought a sodastream for my daughter and am new to the forum as well
BUT I wanted to chime in - I have been doing research on making my own soda syrups and have found quite a few sites that say add 1 to 1-1/2 tsp extract to simple syrup (made with 1 c water and 1 c sugar) - now that is for vanilla syrup so it may vary a bit for root beer but it IS a starting point.
Also, a google books search (searched for "simple syrup extract soda") found old soda shop recipes and it seems like 1-2 drams of extract per pint of syrup is the common ratio for various extracts (1 dram=.125 fluid oz= 3/4 tsp (approx.)) which is about the same as the above amount.
Around 2 Tbsp syrup for a liter is what I'm starting with.
Hope this helps