I made my own out of some scraps of wood and a tiny bubble level, I can post pics a little later when I'm home. It's not that impressive but it gets the job done.

Definitely, it's terrain and tire dependent. I try to stay between 10-25% depending on the terrain/weather. For races/events I'll often overthink and add too much pressure on race day - it probably ends up faster but less comfortable. Most of my casual mixed terrain rides are such a smaller percentage of gravel overall that I stopped messing with pressure/sag and just ride it at my road pressure/sag as the segments are rarely longer than 10-15 minutes and mostly flat/smooth.

I've only been really paying attention to this for a few months. I'm still experimenting and figuring out just how much change in sag makes a difference in tire feel/speed/reduction in pinch flats. I think 2-3% is noticeable but I'm still not sure it's worthwhile to be so quantitative.