I thought "scribe" from your description however I used one of these
I have several of different lengths and they can be set by sliding the tri along the ruler to any depth you want.
Set it at 3/8 inches, Hold your pencil and your mouth just right and you'll get a 3/8 in scribe. Pull the tri-square and pencil along at the same speed and there you have it.
I've seen my Daddy do the same thing with a piece of scrap wood. Position your thumb at the proper distance on the scrap (3/8") Place the pencil at the edge of the scrap then run your thumb along the edge of the material to be scribed and whoop de do a 3/8 " (well reasonably close) scribe.

If it's not broken, you're not trying hard enough.