Thanks, dabac.
I've had the bike a couple of years. I live in a hilly area so need the low range to be pretty low. A friend built up a similar mixte conversion and used a 46 tooth chainwheel which she says works pretty well for her.
Typically when I ride it as a twelve speed I use the middle gears in both high and low range. I resort to the lowest two gears on hills when necessary but can go up smaller hills in 3rd or 4th.
The Sheldon Brown site is brilliant - I'll check out the gear calculator before I talk to my bike shop
Cheers
Fergie