Some of this might get into a lot of nigldy piggledy. . .you might want to make a rule for rule's sake despite common sense, as it may get a bit overwhelmed.
Simultaneousity vs orderly movement by turn create many conflicts.
And you can move and attack without declaring a charge. . .you just move, then attack. (It implies you are not using the move as part of the attack.) as such you can move up to 50% action and still attack. There is an actual difference between:
"I run up 50% move and throw my momentum and weight into a 50% sword slash on the orc."
vs
"I run up 50% move and stop, making a 50% attack on the orc."
Similar overall appearence, but the first is a charge, while the latter is a move and attack.
Really digging into all this leads to all sorts of "But what if. . ."
Like:
Joe has disengaged, and is running from Sam. . .both a humans, both are moving at the same pace, and Sam is moving to keep up with and chop Joe as he flees. . .there is no relative movement, so it makes no sense to give any charging OB bonus to Sam in terms of overall momentum gained. . . .on the other hand, this will mean that Sam will end up with an OB penalty (-% movement to OB). . .and Sam will get the benefit of a rear attack on Joe, unless Joe is fleeing sideways or backwards from Sam. . . .I wouldn't really call this a "Charge" at all, it's a persuit and attack (i.e. just movement and attacking).
Joe and Sam are running along two parallell walls, 5' apart (Makes me think of many a ninja anime movie), both are moving, and attacking. . .neither is charging, again seems to be just moving and attacking.
Joe disengages and runs from Sam. . .Joe chooses to move 40', Sam chooses to move 50' to blow past sam, and chop him as he passes. . .this might be a charge. . .it's a move-by attack. . .but do you give a bonus? Do you give one based on the 10' difference or the full move. . .tough call.
Joe disengages from Sam and runs . . .Joe chooses to move 40', Sam makes a full move of 100' and chooses to do a ram/tackle on Joe. . .an even tougher call.
In the end, if you need a fixed rule, I suspect just making a single call like Rasyr's is best, in that it avoids needing to make spot choices constantly based on the details. . . .or if you want constant variation on situational variables, then you either need to come up with a slew of house rules or just rule on the fly as you go. . .mostly that depends on your GM style.
I find the end result of the way the mechanics work is that you only get one chance to catch someone fleeing with a melee attack, unless you have a drasticly higher move than them, or you have enough of an endurence advantage to make a long chace until they stop running, or the terrain is such that you can corner them.
Since if you chase them, and make a 50% attack, and miss, they will then pull away from you. . .if you can't close that 50% move gap back up, you never get to make a 2nd attack as you can never pull back into melee range.
That may or may not be realistic, but IMO having a slight bend in the rules in favor of "Run away!" is a good thing, as otherwise a PC in a bad situation is just dead. (Similarly, your NPCs can never run off to bother the party another day.) realistic or not, leaving the rules in favor of making an escape tends to work out in the best interests of the long term story. . . .and if you really want to nail someone fleeing, Missile or EAR them. (Or be mounted/flying when they're on foot)
