Problems building a working magma forge in current version of DF

I am having problems building a working magma forge in my fortress (using the most current version of DF). I have built magma forges in prior versions and am aware that I need to place the "darker green" x in the magma forge template over the hole leading to magma in the Z level below the forge. When I build a forge in my current fortress it apparently builds ok, but when I try to set jobs for the completed forge it says it needs magma under the forge. I have excavated a large open area (approx. 50x40 tiles) one Z level lower in my fortress that is still filling with magma (currently filled to 2-3 depth) from a connected magma pipe. Do I need to wait until the magma fills to a higher level than 2-3, or are there other requirements I am missing to get the forge to work?