When using 18- 20" of wire I had fewer bite offs last year. The shorter the leader I had them winning their freedom when they bit through the mono. It's my guess they were just inhaling the bait and getting past the knot.
Alben, I like to cut my wire to about 15". After going through the sleeve twice on my crimps, I end up with a 14" leader.
I feel like you should go as short as you can without letting the wahoo reach the mono. I've done really well on leaders as short as 10". But on other trips, the wahoo swallow the bait down, and can reach above the wire. 15" seems to be the best compromise length for me.
I don't understand why wahoo T-bone a lure, but will attack a bait from behind, however I learned the hard way on my last trip and lost a few fish when the 'hoo gobbled on past my wire leader and bit through the mono...no more short leaders for me.
A couple of hints--the average 3700 tray is 14" lengthwise, so if all else fails you can use that as a measurement guide (you can also use a piece of old mono as a template for length). If your leaders are made of solid wire, don't store them bent in a way that they can get a kink in them, they're useless after that. They are also often good for only one fish because they get deformed after that. And do indeed have some "extras" available; they don't go bad and you don't want to be making leaders in the middle of a bite. You don't need a fancy hook, but you do need a sharp one. Use black rings, not shiny ones in making your leaders (the hoo are attracted to shiny stuff and might bite the rings instead of your bait). No need for swivels, the plain rings will do fine. If you make your own, its an absolute to remove the tag end of the wire by bending it back and forth rather than cutting it--it takes longer, but it doesn't leave the kind of sharp end that pierces a deckie's hand when tending to the fish.