When you cut your water, your body only stays 'fooled' for so long. Obviously you can't maintain such a condition for 48 hours, but you can do your best not to have any serious rebounding occur. I've always heard people talk about picking which show (if you're doing two close together) you should truly aim for peaking out.
In your case, while the Judging should have some influence in the overall placings, I would guess that the prejudging (Thurs) should be your target as far as water/carb/sodium manip. After that, just take things easy. Most people usually whine that they look better 2 days after their show anyway -lol.