HP-71B 4Kb (CMT32k FrontPort) Memory Module "Battery" - FRAM

[Hans Brueggemann]

from my experience, 10 µA is absolutely on the safe side, you probably get away with just 1 µA.
so, basically you just adjust the size of the cap to the allowable delta_V for, say, 10 sec. of "shuttling" time. with Vnom = 4.8 V and Vmin = 3.6 V you get (4.8 - 3.6) V x C > 10 x 10 µAs. thus C > 83 µF.

one caveat, but i'm just guessing here: with digital inputs left unterminated during shuttling, chances are that parts of the module's input logic might go into linear operation and thus increase the current drain from the capacitor.
as to what your HP-71B's opinion on the possibility of file chain corruption is... well, you know the answer!