Back in my Fuji S2 Pro days when I was shooting a lot in jpeg mode I took a custom white balance in the backyard around 4 p.m. in the summer months and I had great success with that. Our house was located at the very foot of a very large Hill covered with Douglas fir trees so there was a lot of green in the ambient light.
One unusual white balance target that I used to use was white clouds in the sky, thrown well out of focus.
I now very seldom take a custom white balance, but often input known Kelvin readings.
Back in the early 2000s, before Raw capture was Universal, there was a lot of attention paid to white balance issues, targets, and methodologies. Now with the increase in sophistication in image processing software and the more near-universal acceptance of shooting in raw, there is a lot less attention paid to the subject. My experience is that a custom white balance can greatly improve your results based upon the actual shooting environment. When I say large hill covered with fir trees, I mean about a two thousand acre forested range, not just a 40 by 50 Foot Hill. I remember one UK fellow whose neighborhood white balance experiments inspired my own. He took different white balance settings at different times of the day and I found that in June,July,August , and September,in my neighborhood, that a reading taken around 4 p.m. off of a neutral white Target gave outstanding results throughout much of the day, but the last hour of the day I was better off using the S2 Pro's automatic white balance setting.