From a pure light gathering POV and taking some simplifying assumptions...
Going from 45% QE to 68% QE gives an increase in sensitivity of about 1.5 times (actually 1.511111111...) That's equivalent to an aperture increase of just under one and a quarter times [sqrt(68/45) or approx 1.229].
So, 4" aperture at 68% QE is equivalent to about 4.9" at 45% QE, 12" aperture at 68% QE is equivalent to about 14.75" at 45% QE, etc.
Cheers,
Rick.
