I generally use around 10% of extract, just a round number and anywhere from 5% up to 20% appears not to make a huge difference, but 10% appears to be close to the sweet spot.
Too much and the beer tends to be a bit thin, not enough and you don't get the real Bananarama I'm looking for.
A matter of personal taste but the best hefeweizen I have made was 50/50 Floor malted Wheat and FM Bo-Pilsner both from Weyermann, 10% Dex and WY 3068.
I'll have to give the 70% Wheat, mentioned in the article a go, BIAB makes very high Wheat fractions just as easy as any other. Not always the case in 3V or modern recirculating systems, unless you mill very carefully.
Mark