So, thinking about this some more, I come to the following conclusion:

Leaving the spacer on the shaft, means that my primary ring would be the 39T, with the 50 used mainly when I need a higher gear at the top end.

If I remove the spacer, that shifts the 50T into the middle, making it the primary ring, with the 39T serving as the granny gear.

I'm assuming that a standard double uses the latter configuration, i.e., most of the time is spent on the larger ring.

Have I got that right?