Nice plagiarism

... made me realise I missed aswum's initial post.

..... next time I'll just make a template and use the router table to cut...
Totally agree, engraving over grain is like sailing over the Rockies... 
Using this as a guide,

would suggest cutting the green and the outside of the red (with the blue representing the grove)... that way you could use the red-blue "offcut" to centre the green on the box... something I always remember after I cut the routing template.

If your first cut is too short... Take the second cut from the longer end... LBD