It sounds like you're looking for a ratio from an engineering standpoint. A kitchen knife is not a bridge!
I believe that a hidden tang knife needs that tang to be as long as possible. If I'm making a 3" bladed hunter with a 4 1/2" handle, I like the tang to be close to 4" inside the handle. Same if the blade is a 9" fighter.
I believe that if you make the tang 2"-2.5" as Stephan suggested, then attach an extension you'd be fine. I usually hard solder an extension on with a dovetail joint, but it can be brazed, or before heat treatment welded.
A cross pin through the tang, whether through the original blade material tang or the extension, is used as a mechanical reinforcement to your adhesive. The pin won't take the place of the tang or make up for a short tang. If you use a good epoxy like System 3, West System, or Acra-glas you probably wouldn't really need a cross pin on a kitchen knife unless you just like the look.
Here's the problem with a short tang. If over time your wood handle moves around and cracks, or if it had a natural defect to begin with, you're handle is going to come apart probably at the end of the tang. There was a rather big name maker years ago who did just that. He was saving money by not using so much 440c. When some people actually used his knives to clean game the handle broke right at the end of the tang. So put on an extension and give your handle some reinforcement.
David
