I'm not the norm on this one, but I find that eating before kava doesn't impact the experience. And that eating after kava doesn't impact the experience either.
I generally don't drink kava IMMEDIATELY after dinner (I tend to wait 30 minutes or so) but I've really not noticed any negative effects with food/kava excepting maybe once when I ate some starchy food in the middle of my kava session and sword that I felt some attenuation of effects for a while.
Food only stays in your actual stomach for 30 minutes, and then it's all in your small intestine. If kava is absorbed primarily in the stomach, you should be good to go after 30 minutes of resting. If it's absorbed more in the small intense (anyone know?) then you're talking about an hour at minimum and probably more.
The 40-60 minute effects onset has me thinking that it's probably a combination of stomach and small intestine absorption.