As I had to use my AMEX for all purchases, and as it is a charge card, and I have no idea how high my limit would be, I am not able to limit myself to 20-30% of the limit (the way it is frequently recommended).
As stated above, your charge card isn't relevant to revolving utilization. However, when you do get a revolver you can certainly manage reported utilization by paying down your balance prior to statement end. While 30% is the generally recommended max, you'll probably see better scoring at even lower utilization levels.