It really depends on how fast the player is moving. Bunny hops can build up speeds in excess of 70mph, so obviously you can jump further than jumping from a still position. Also, it depends on the player's packet rate, z-axis position, gravity, and other things as well. It's not an exact science. Trial and error is usually the best way to determine these things. Just build a couple ledges and see if you can jump from one to the other.