I've searched around and not found any useful information on this topic.  Can anybody tell me as to whether a .40 cal Carbine should initially be zeroed at 25 yards or 50 yards?

For some, I think it depends on how close they want the Zombies to get from their given firing position, before said Zombies are labotomized.

If you shoot the lightest 40 bullet, (135gr?) chances are it will be the flattest and more forgiving in bullet rise and drop at various distances.

Set your zero to your intended firing distance with ammo intended to be used - this applies in all firearm types.

Personally I'd go with a 100 yard zero, as even shooting a 40 pistol at that range (just did so yesterday with 180gr's) the fixed sights on my Sig put me right on target.

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I've seen advice about 9mms calling for zeroing at 25-50 yards.  While the .40 is more powerful and can be faster, I'd go with that range.  Half a football field away is more than adequate for knocking down zombies without having to wiff their guts as they splatter.