It’s not reckless.
And it works.
Don’t save time if you don’t want to. I’ve shown this technique to others and they love it. Do it the hard way if you prefer.
I’ll say it again. A darkened negative image of your grayscale diffuse is a great starting point for your Power Map. Darkened gray scales of the diffuse also work great for the Environment and Spec Maps.
The funny thing is that your method is actually harder than mine.
My method for spec maps is simple.
Bottom layer is entirely the desired shade for painted surfaces (dark gray).
All other elements are then set to black or white on their individual layer and that layers opacity is adjusted to create a gray shade for the drsired sheen. Simply decide if an element needs to be more glossy(white) or flatter(black) than the base paint.
For example, for smoke stains I change the smoke from sooty brown to black. For oil streaks, I change the color from dark olive to white and reduce that layer's opacity until I get a light gray that I want. I can easily make adjustments just by using the opacity slider.