It'll probably be a bit lower unless you do a cereal mash. I'd prefer just to add more wheat berries though... I'd figure about a 50% efficiency for it... so I'd increase the amount of wheat berries.
This would work out to 33% Pilsner Malt (actual weight unchanged) and 67% wheat berries. Keep in mind that it's better to overshoot... you can always dilute with water if the OG is too high.
The main thing is to make sure that the pilsner malt you're using has enough diastatic power. Anything lower than 100%, and I'd be substituting white wheat malt for SOME of the wheat berries... possibly equal parts wheat berries and pilsner malt. Or just add some amylase enzyme powder tobthe mash.
One thing I should mention is that a pretty traditional ingredient for witbier is oats, and IMO it's pretty important to get the proper mouthfeel down. I would strongly recommend a grist with about 10% oats.
Lastly... what is your mashing system like? Traditional 3-vessel? Recirculating? BIAB??