hm...
I am hoping for a system which you turn/spin/slide which both does the calculation and leaves an immediate record of the mb you are supposed to be flying. This would mean not having to write down every single change separately.
Ie (please ignore the figures, for illustration only):
ATC give you 1013, you convert to 29.92 and put it in altimeter.
A little later, when you make a call, if you read the inches off (they're pretty small), and then calculate back, the chances are you'll tell 1012 or 1014 to ATC, with them then having to correct you.
Over months or years (but perhaps even hours) this would get really tedious!
An mb altimeter is on the list, but it's not a cost-free change!
Sam.