The rules for partial cover in Total Warfare allow the target to decide which hex applies when an attack travels between hexes. However, on page 84 of the sixth printing, Tactical Operations: Advanced Rules introduces 25% cover for that scenario.
While this rule is quite clear when there are actually two hexes the attack is traveling between, I'm not sure how this should be applied when one of those hexes does not exist due to being outside the playing area. An example of this in MegaMek is attached to this post.
MegaMek's implementation of the rule appears to assume that hexes outside the playing area constitute level 0 terrain, as the Wasp is granted 25% partial cover from the Clint IIC's attacks. Is this correct, or should 50% partial cover from the adjacent level 1 hill apply instead?
