With some practice, you’ll just be able to use the brake any time, and at (almost) any distribution of braking through the pedals vs. with the brake.
One thing you will need to learn, as elpuebloUNIdo said, is to seperate the braking finger from the holding of the handle. It really depends on your setup, but with mine, I have my index finger on the brake, and the rest on the handle.
I started by just using the brake on longer descents, and smoothly increasing the brake pressure until I was at the point where my legs were only used for balance adjustments, and held it there. Essentially the same as you described. When you are comfortable with that, play around with it. Open the brake a bit, and go faster, tighten it and go slower, both without doing much with your legs as far as breaking goes. At the same time, you’ll likely pick up on how to balance using the brake. It’s not really rocket science, you are doing the same thing as you would do with your legs: If your weight is to far backwards, you need to brake more, if you are to far forwards, you need to brake less. I personally think a decently steep downhill, that you can still control with just your feet is the ideal place to start using the brake. On flat ground, you need to be a lot more sensitive with the brake, on downhills, you can make a few more mistakes I think.
Also, on almost every dismount, I used it to stop. Yes, you’ll fly of forward a few times in the beginning, that’s why you should start at a slow speed. But after a while, you will learn to lean back, and then engage the brake. Once that is second nature, the whole “feeding in the brake very carefully before you have reached the steep part” will become uneccessary.
A few years later, and I don’t think about it anymore.
Someone has an UPD on the trail in front of me? Grab the brake.
Ride over a drop, and I leaned to far back? Grab the brake.
I’m worried the brake is overheating on a long descent? Let go of the brake for 15 seconds or so, and get back onto it.
Brakes have a learning curve to it, but it’s so worth it to take it.