Okay, I figured it out (this is called "learning curve"...

The symbols that you create in EL automatically have the tool path applied unless you tell it not to engrave it, or delete the symbol, after you create the proper tool path.

So, although the tool path using the "fill" was set correctly, taking the width of the tool into account, the symbol that I used originally was not - it was not taking the tool width into account.

So, I deleted the symbol, which just left the tool path, and it engraved at precisely 39mm.

Thanks for the help.