I carried one (Jepp CR-?) for years, but never found a use for it, that wasn't already displayed somewhere in the cockpit. In my current flying, I sometimes need to do ETPs and it might be useful there, BUT the Honeywell FMS can do it or I can do it on a electronic calculator just as well and compute my expenses. I love E-6Bs , though and the two Pickett Slide Rules, I have. Might even be worth putting on Ebay