Yes, keyboard just above the knees. It's similar to the angle you'd get if you were slouching and your forearms were resting on the desk. Generally quite comfortable.
I think raising my hands up higher (90 degrees as you say) would actually be a lot worse. Never understood that aspect of all the more "real" laying desks.
I should note that I purposely don't use a mouse with this setup, because it would end up being quite far away and would need to be on a solid surface.
I use a very similar setup, and I use a mouse because I've never found a keyboard I liked that includes a touch pad. I've simply taken another monitor arm + a laptop holder attachment. Flip it and put a firm pad on top and you have a mouse pad holder that you can position in 3d exactly where you want it.
I would love to use something with a TrackPoint, but I have an irrational fear of committing to a non-fullsize keyboards. It's regretful that they don't build different keyboard variants or license the tech out, but I assume the market isn't large enough to warrant more offers.
I think raising my hands up higher (90 degrees as you say) would actually be a lot worse. Never understood that aspect of all the more "real" laying desks.
I should note that I purposely don't use a mouse with this setup, because it would end up being quite far away and would need to be on a solid surface.