That's a good point I guess... though the exercise was interesting in itself 
It seems I forgot to take the voltage drop into account, with
C = I × Δt / ΔV
if I want to limit ΔV to 0.4V (2.2V towards end of life - 0.4V = my BOD setting) I end up with a 1750uF capacitor... not that attractive.
Franck