Okay, How is this (for a first draft)
Control Zone (CZ) - This is a 5' area to the sides and front of the character/creature, approximately a 180 degree arc. The next 60 degrees on either side of this control zone are the flanks, attacks from the flanks gain a +2 modifier. And the last 60 degree arc, directly behind the character/creature is the the rear, and attacks from the Rear gain a +4 modifier. Large humanoid creatures have a 10' Control Zone and Huge humanoid creatures have a 15' Control Zone. Non-humanoid creatures, especially those that are Large and Huge will have similar Control Zones, but the actual shape is likely to depend upon the shape of the creature. Just keep in mind that the Control Zone rarely extends behind the creature's shoulders, and that the rear zone will always be directly behind the creature, with the remaining sections on either side being the flank zones.
Opportunity Attack (OA) - This is an attack that happens outside the normal initiative cycle, but it still counts against available actions for the round. A character who has multiple attacks (high skill, multiple weapons, etc.) may use all of their attacks for the OA or they mayuse just one of their attacks for the OA and then take the rest normally according to initiative procedures. Opportunity Attacks are triggered by a potential foe entering or crossing a character's Control Zone without the intent to attack the character.
Disengaging from Melee - To safely disengage, without triggering an OA from the foe, a character must spend 1 AP for Disengaging and then move, spending other AP, out of the foe's Control Zone.