It appears that it's about 3/8-inch below the 3/4-inch plywood core; I highlighted the line between the core and glass for better visibility. It is also 3/8-inch in the center of the hull where it's not so complicated with running strakes and chine stiffening. Your boat may vary...
Tom Herrick
If anyone can tell me why this pic didn't come through in the post, please enlighten me. Seemed pretty straightforward, but...
Not knowing what you're running into with your boat, I thought I'd post this as something to look at if you're having transom/sole problems:
On my boat, I found that the last two feet of core and the glass below were bowed down about an inch. I've heard from Larry H and others that this is classified as "bad". It can make the boat handle poorly if it's out of plane by that much. If you have a long, say six-foot, straightedge you might want to lay it fore to aft in the cockpit and see what's what. I didn't notice the sag, however, until I cut the transom off.