Is that 20% buckwheat added to wheat flour ? or 20% of any of the mix of flours in gluten free ?
The 25% is kind of the concensus on vegetarian sites for adding buckwheat (Haven't checked Larousse), where they're playing around with gluten free baking too, to be added to wheat flour but still makes baking that has both the right texture, mouthfeel, looks appealing, smells right and tastes good.
The rye bread I checked last here had 65% rye in it, I admit it looked stodgy. Oat bread made only with oats is pretty heavy too.
Cultural mores and comfort foods are I think set in childhood to a large extent.
If I toast oats for cranachan my son comes home and he's straight into the kitchen
He loves that smell. The other one it's roasting hazelnuts that gets to him.