I fitted a boost gauge a nearly two years ago, but I never got around to fitting the light bulb

I'm going to use the power feed from the widget, after the fuse for the widget, (or should I wire up a separate fuse for the red wire for the light bulb 12v 3w bulb??)
but where in the driver's side foot well can I get a "good ground" for the black wire.

use any feed with a in line fuse and for the illumination use the cig lighter the yellow and red wire

why not take the power source from the illumination of the dash dimmer switch or the centre console illumination (depending on where you have your guage), that way when the lights are on for the dash your boost guage light will come on as well. Thats how mine's done (albeit by the previous owner)

its not difficult to do, as you dont need to remove the clocks & comes on with the other appropriate cabin illuminations. - its right next to the widget source you were intending to use as well! just use an electrical tester to check which wire is live when the lights come on!

with all things, the devil is in the detail, it may seem trivial but its things like that that really make the difference.