Whatever gain ratio you choose..also keep in mind that fiding fixed on hills will make you stronger and will allow you to develop faster cadence abilities. So if you start out with something that seems too hard, it may seem like the right gear later. Or if you start out with something that feels just right, you may eventually want something higher.
I started riding fixed with a gain ratio of 5.1 and that seemed about right...but after awhile it just seemed too low and I moved up to a ratio of 5.7 which seems like the gear that I'll probably stick with.