Brompton owners

I was thinking of going for 42.1"/56.2"/74.7" and then ride in 74.7" most of the time.

but the options are:-

std - 47.9/63.8/84.9
-12% - 42.1/56.2/74.7
-18% - 39.1/52.2/69.4
+8% - 51.7/69.0/91.7

I guess I could gear up (+8% instead) and use the middle gear (69.0") around town, leaving the lower gear (51.7") for emma to use on hills/pulling away at lights and the higher gear (91.7") for me to make the jamaica road roadies cry when I've got fresh legs

I would recomend your standard gear to be the middle... this way as you say you can step it up or get up some of the more cheeky hills.

automattically putting yourself in the top gear seems a bit counter intuitive to me