I've been meaning to mess around with reference lines in greater quantities. You can go in many different directions with this concept. Specific to this trick of creating gradients, the basic rule of thumb is to work from the outer edges inward. It's mostly about saving time with the mouse clicks. I thought about doing 40 or 50 reference lines on each axis, but have I mentioned how lazy I am? I really do like this analytic concept, though. I would love to see someone create a almost-real-gradient using reference lines. The software can certainly handle it.
In the example below, I placed gradient reference lines for 95,90,86,83,81,80, and 75% (and their counterparts). You can adjust the occupation slider to update the visual.