How thick ApplePly should I use for a baffle? Is ApplePly truly measured in inches?

[MikeRocker]

I am putting together a 1x12" guitar amp extension cab, and I think I want to go with ApplePly for the baffle. Should I use 1/2" or 3/4"? I'm planning on using a heavy speaker, maybe an Eminence Swamp Thang, which weighs just over 11 pounds. Also, is ApplePly truly measured in inches? I ask because I found out that Baltic Birch, while labeled in inches, is actually metric.

Thank you for any help!