and what about the 3V side of things
Is that 6 / 0.8 = 75R resistor.
Can I use a similar calculation for working out what resistor I need to connect to the base of the transistor?
Seem to remember reading that I had to ensure they have a commom ground?
Yes the current you want in the collector is the current in the base time the gain of the transistor. If you don't know the gain then guess at say 50, if it is an under estimate it doesn't matter.
why isn't 6/.08 = 75R (accepting that I wrote .8 in error)
Yep, that's it. I've gone