Thanks for your feedback. I can definitely look to adding some new functions. The only potential downside to using F16-F19 (aside from some potential technical road blocks) is finding the screen real estate to create matching on-screen keys, but of course I can figure out some solution to that.
There are, however, similar keys available currently. The UNDO key is somewhat akin to the 'LAST X' key (clear, up arrow), however not totally equivalent.
The DELETE key is perhaps similar to the rotate key (down arrow) which drops the a number off the stack (putting it back at the top).
What would you expect the DELETE key to do? Does it change the current bottom number to 0 or does it drop it off the stack, replicating the top number of the stack?
Perhaps the delete key (backspace) would be a logical choice for one or the other of these functions?