I can confirm what Hennns says: it's calculated as a percentage of the armies which could
move. However, I want to make a small note regarding to rounding. Just tested this:
- 50% of 1 is 0
- 51% of 1 is 1
- 50% of 3 is 1
So it's a straight round, rounding down on a perfect half (I tested the 50% of 3 to test for Banker's Rounding: http://en.wikipedia.org/wiki/Rounding#Round_half_to_even
It's a general rule of WarLight that an order moving zero armies (for whatever reason) does not take up a "slot". This is necessary to prevent people from stalling (a poor man's Order Delay) without actually using armies to do so. That rule also applies here.