yeah I use whiteboards first, and often ignore the pixel density initially. I also just find that is a good size for low fidelity. Sometimes I'll use progressively darker %gray markers to go from almost-invisible to a more concrete/defined concept. It's great to be able to make a broad 40% gray line and say "this is the title of the panel" and nobody questions it because everything else is just gray, too - and the darker shades will be visually elevated in some way later. Purifies the UX, and yep - keeps it all about ideas.