Not really that lame at all. It is something you want to be able to do on your own, and if squinting helps than you are one step closer to that than if you are using some other tool.
Color has hue, saturation, and value (or tone.) What you are doing in your minds eye is ignoring the first two to extract the third. But there is also relative color, and the tone you get from one color will appear very different depending on the colors around it.
The grey scale is a just a tool that has done the first step for you, so you can compare side by side to get your value name, usually in 10% steps of the percentage of darkness where 0% is pure light/white and 100% is pure darkness/black.
But really, the faster you can ween yourself off of using devices to interpret tone values the sooner you can do it purely in your minds eye and the better off you will be. It is the tone you choose to apply along with its neighbors in the artwork that matters, not its name.