I have a north facing window in my studio directly next to my easel, but since I live in Oregon where the sun isn't terribly reliable, I also use a florescent light directly above my easel. I bought a nice fixture (the cheaper ones buzz and flicker) and installed 4 bulbs. Unfortunately I found my paintings were often too dark, so I removed the bulbs one by one until I had just one left. This seemed to be perfect. Then I met an artist who used two bulbs (one warm, one cool) and so I tried that. I like it, but with 2 bulbs again I found it too bright. My solution? I covered parts of the light with tin foil to cut down the light (so you know I have an 8 ft ceiling). This is perfect for me now ... until I find something better.
I use Philips, Alto, T8, Natural Light (5000K) and Cool White (4100K). The balance is a little on the warm side, but I reason most paintings are going to hang in a warmer light, so it behooves me to paint in the same. If you have a different opinion I'd love to hear it.