When you're at a sewing desk, your arms should sit approximately flat on the desk. Right angles to your body. I won't give a measurement, because I'm almost certainly not the same height as you are.
I looked at the option to use the foam, but decided against it. Why? At the end of the day, before adding foam, my desk was too tall. My shoulders were already around my ears, and I had "Dracula arms". Adding the foam would have made my ergonomics worse, regardless of any chair I could use.
So instead of going UP, I went down. I recessed my machine into my desk. Best decision I ever made. If you have the room, pick something up from a thrift store, or a government surplus (these are my favorite, the furniture is really sturdy, and really cheap) and hack about. I found that even once I'd recessed my machine that it was still a little tall, so I shortened the legs. When you pay $10 for a desk, you can do that without feeling like you're throwing a bunch of money away.
ETA: These are 2 different posts. The post got fractured when it was copied to the tutorials forum.
http://www.quiltingboard.com/tutoria...e-t216324.html
http://www.quiltingboard.com/main-f1/height-adjustable-desk-can-recess-machine-its-true-t216312.html