Dialing in the grind at least to the first approximation of where it should be on a pump machine is not too hard in the sense that you can monitor if the shot when pulling a double is producing two ounces of espresso in twenty four to thirty seconds. If it is pulling too fast you grind finer and if it is pulling too slowly you grind coarser. (Of course it is more subtle than this because you can also produce these results by adjusting the dose – the quantity of coffee you are using etc) This at least gets you in the right ball park. With a lever machine this does not work because the shot starts when you start pushing the lever down and it stops when the lever reaches the bottom. If you push it faster you will produce the shot faster and if you push it slower you will produce it slower, of course. With a spring lever, the spring controls the rate so this applies only to manual levers such as the La Pavoni and the Olympia Cremina.
How do you then know how to grind coarser or finer? The basic idea is that you want to press the lever down with roughly thirty pounds of pressure. This is hard enough that you are exerting some effort, but not so hard that you are straining. More precise estimates can be obtained using a bathroom scale to figure out what it feels like. Obtaining the exact pressure is not necessary, but it is nice to have something to shoot for. At this point you can then adjust your grind so that a shot takes 24 seconds when exerting thirty pounds of pressure. Within that some people like to grind finer and tamp little or not at all and some like to grind coarser and tamp at thirty pounds. The more crucial thing there is that you are consistent than which of those two paths you choose or if you choose something between the two.