I'm usually using 25-40% of "something else" and rest is regular wheat. The stronger the wheat is the easier it is with higher % of everything else. But my favorite is 67% wheat and 33% spelt. That's the best, the dough is good and spelt gives a nice flavor.
Experimenting really helps. It's great you're testing the buckwheat and nuts, buckwheat can be tricky but when you know your and buckwheat's limits, you can find the perfect balance for yourself :)
Normally I've used a machine to knead the dough. Something like this:
However, usually a more expensive machine is needed for reasonable dough's as most machines can just break down when you try to do a proper dough. This method is kind of easier, as you only need to run the machine at the beginning and then you can have a longer pause. With stretching (the dough, not me ;D ) I can't leave the dough for that long because I need to stretch it.
Thank you for this long and profound answer. I am still at the beginning of my bread journey and a kneading machine is not planned in the near future (oh my ... they are really expensive). I will experiment with the stretching method and different kinds of flour (the buckwheat worked fine, but I only used a tiny amount in comparison to the other flour).
Next thing I have to improvise/buy is something to get steam into my oven without destroying it :-D I want to be able to bake bread without a pot.