The manual states directly that as soon as the wing tanks are less than full, you may not be carrying more than 1000 lbs (454 kgs) of fuel in the center tank.
So, as soon as you START to use wing tank fuel with more than 1000 lbs (454 kgs) of center tank fuel, you're outside the ZFW limitation.
IMHO get the thing on the ground as soon as.
That's the way I see it!
Cheers...FD...
