You are right that in older versions of Blender they used a different way of measuring it as percent of a blender unit. This got confusing as you went DOWN to increase detail so they changed it (I think version 2.78 or 2.79) to be divisions of a blender unit. This is only for fixed detail though. I believe relative relies on how how many screen pixels you are allowing you to zoom in, create finer details, and then zoom out again. This will depend on your personal workflow for which you prefer.
It will take some practice to decide where you should be in the number range and which mode you are working on. I would say create a sphere or plane and then practice making strokes on it going through a range of settings and try to see how it works.
If you find that you are getting too detailed and things are becoming lumpy, you can turn on collapse/divide and use inflate at a light strength and give a stroke to lower the resolution and then a quick smooth.
