Robin, I use .375" (3/8 Inch) thick HDF fiber-board. Makes a lot of dust while cutting,
On some monograms I used a .250" end mill, others with lots of detail I used a .125" end mill.
Using the .250 em three pass at .125" each.
Using the .125 em you best use lighter pass, I did mine at .060" per pass
To get what I think is a better looking piece, I always change the text into vectors, and do a lot of node editing. Trying to make all the curves as smooth as I can, and to delete as many node as possible.