The type 'E' screen is a popular one. I have one each for my two Nikon F2 bodies. It's a more open screen in the sense that there is no distracting focusing "aid" right in the center of the field of view, obscuring the composition. And for asymmetric-seeing people like me who couldn't hold a level building or horizon if our life depended on it, it's gridded lines are a life-saver.

Just learn to focus by looking carefully at what appears in focus, just like looking at any other ground glass camera. It's really not a question of guessing, but rather one of careful inspection. There is no missing trick to this, although there are magnifying eyepiece attachments on some Nikon models to help out a bit. These, however, are usually reserved for critical macro-level focusing, not general photography.

Many Nikon users will tell you that a Nikon equipped with the plain, unadorned, non-metered prism together with a plain, unadorned screen consisting of nothing more than the ground glass itself is the purest form of 35mm framing and focusing there is. Heck, even my 8x10 ground glass has faint lines.

And enjoy that FM2n. It's a marvelous Nikon. I keep meaning to pick one up for myself.