I've always been shown to use the RPM as guide to leaning and listening to the engine, i.e. pull the mixture out until you hear a tone change in the engine or a drop on the Tacho then push in slightly??. a CHT gauge is also a good guide too.
You're doing just fine. That tone change is actually the onset of rough running, where one of the cylinders is starting to run too lean, and not contributing the same amount of power as the other cylinders anymore.
I regularly fly a R2160 and the induction system is such that the onset of rough running actually happens at peak RPM. The POH doesn't even have instructions for economy cruise, just for best power leaning - which is simply considered identical to best economy.
We tried it but were never able to achieve a smoothly running engine when running leaner than peak RPM.
As far as the CGTs are concerned, yes, they are a good guide, but you've got to take into account that they lag behind tremendously. So if you want to use CGTs for leaning, make a slight adjustment, give it a minute to stabilize, and then make another slight adjustment. I have seen people using precisely this method in the climbout. The engine was doing 100% power (0' AMSL), with full throttle and max RPM (this was a C/S prop). They were carefully monitoring the cylinder CHTs (per-cylinder CHT indicator) and were leaning until the hottest cylinder ran at - from memory - 450F. But it took them a few minutes to reach that setting. They also had the advantage of having a fuel injected engine. It's one example of getting a more sophisticated engine and engine monitoring, allowing you to do more sophisticated leaning.
Note that with that method you're not reaching a stochiometric mixture. You're still using an overrich mixture to help with engine cooling - and since the engine is producing more than 75% power you need that help. But you're running less rich than full rich, so you are already saving some fuel.