Have you played with Sheldon Brown's Gear Calculator? If not, I recommend it. Most people set it to give results as "gear inches," but it really doesn't matter. Figure it out and get used to it; it becomes a very useful tool.

Enter your chain ring sizes etc and calculate; you probably end up with something like this:

50.1 88.7 92.6
46.8 82.8 86.4
43.9 77.6 81.0
39.0 69.0 72.0
33.4 59.1 61.7
28.1 49.7 51.8

Before you get confused, just ignore the biggest chain ring (third column). Now, looking just at the first two columns, and noting that two of your gears measure 50", you basically have eleven distinct gears; let's pretend they're evenly spaced, and number them as follows:

6 11
5 10
4 9
3 8
2 7
1 6

To this you can add the biggest chain ring for half steps, i.e. 6.5, 7.5, etc. Though, frankly, changing back and forth between a 46 and 48 chain ring is quite a bit less than half of a step.