Run 17 or 18T cogs, you should be set. I used to run 51x16 its tough to ride up the hill with my massive body and ****ty heart.
Now I am running 46x16..alot better, but I am going to run 18T cog soon I will tell u what happen. hehe
That is a pretty high gear. If you're used to pushing it on a singlespeed it won't really be much different fixed. But you might enjoy the jump to a smaller ratio if you try it, with either an 18 tooth cog for 75" or a 19 toother for 71". I really like a 69" gear (47/18) though it's not for everyone. Small gears on a fixed are all about power and control---easier to get going, easier to slow down.
The typical factory fixed gear comes with a 48t chain ring and a 16t cog, which, with 700X23 wheels and tires, corresponds to 78.8 gear inches.
The phrase gear inches refers to the amount of effort required to pedal a Penny Farthing Big Wheel bike having, in this case, a 78.8 inch diameter Big Wheel.
The original poster's 50 X 16 setup, with 700X23 tires and wheels, comes out to 82.1 gear inches.
I have ridden an 82 gear inch setup in an mixed urban/suburban/rural community having a dynamic gain and loss of about 1000' with success.
I can't brake going downhill by back pedaling at 82 gear inches.
I personally, in my riding area, consider 72 gear inches the most versatile gear set up for me and my riding area.
I can brake going downhill by back pedaling at 72 gear inches.
50 X 18 makes 73 gear inches.
I presently ride 42 X 19, which makes 58 gear inches, as part of a learning experiment.
Sheldon Brown maintains an online gear calculator.