Hey esk1020,
on a Onefinity Journeyman, the machine will allow you to have the center of your milling bit to go from 0 to 1220 mm in the X direction and 0 to 816 mm in the Y direction (or whatever is set in the MOTORS pages under “Limits”). You can then add the radius (=half the diameter) of your surfacing bit to each of the four sides of the workarea, so you know the maximum surface you can mill with it, but there will be a little region in every of the four corners of the wasteboard that cannot be reached by the bit because the outline that a bit mills is a circle. You can remove that with a chisel or handplane later.
Even when using a very large surfacing bit, you don’t have to fear to mill against the machine’s feet as there is enough space around the machine’s workarea, see Cutting area location.
Note that when you use the QCW frame, the milling bit will not move exactly over the area where the workarea seems to be, i.e. between the four machine’s feet, but its movement area is much shifted towards front because the bit and the Z slider are not exactly in the X axis but attached to the front side of the X carriage. So there is a rather large region that cannot be accessed by the bit in the rear of the QCW frame, of where the t-tracks reach, and where you could also make the wasteboard slats much shorter since neither you can surface that area nor can you mill something there. Correspondingly, there is a rather large workarea where the milling motor and the bit (and the dust boot!) protrude the QCW frame beyond the front, where you will have no support for a wasteboard, so workarea that is wasted, and that you have into account when building a table and more specifically, an enclosure! Because of this, don’t build a table or enclosure that ends at the machine’s front feet. See Dust boot overhang and more links below for exact dimensions.
Note that the shift of the workarea with respect to machine’s feet (and thus the inaccessible, wasted area on the rear of the wasteboard slats when using the QCW frame) differs a bit depending on whether you have the old Z-16 or the new Z-20 Z assembly and of course also whether you have the 65 mm or the 80 mm spindle mount, as each results in a slightly different position of the center of the bit.
I think someone measured the useless area on the rear one day, but I couldn’t find it anymore for this post. Since the exact distance differs a bit depending on the options above, I think you should measure it on the machine (and report it here!)
This phenomenon can only be avoided by not using the QCW frame and making a machine base that has its wasteboard where the machine’s workarea is.