http://dougkerr.net/Pumpkin/#APEX has a good description of how to work with APEX.
I forgot to say thanks for the Doug Kerr APEX link. Based on your message, I spent the night writing a spreadsheet to convert into and out of APEX, something that I struggled with for weeks. (I was using the more common but more complex logarithmic system.)
Anyway, that stumped me for a long time, so I appreciate the useful pointer.