So I chose 75 km/h as an arbitrary speed at which I'd like to be able to actually use 5th gear (on mostly flat road). In other words, the RPM can't be so low that it's either lugging, or not able to produce enough power to propel the car at that speed.
A pretty picture comparing 5th gear / final drive combinations:
The ratio of the 5th gear assumes I could successfully swap the 2nd gear ratio: from 1.894 to 0.528
So, with my tall final drive, I wonder if I could drive down the road at 75 km/h at 1279 RPM. I'm not sure - have to take the car for a spin and see how it feels at 1279.