It seems to me that your are thinking about landscape design from the perspective of horticulture ... as if trying to grow perfect plants to be viewed from all directions, as one might grow plants in pots for resale. Usually, in landscape design one is thinking about how to create architectural works (floors, walls, ceilings and roofs) that are made of plants. In most cases these works will be made of many plants placed in groups so as to touch or overlap. A hedge being an example of a "wall," you can see that the plants are placed closely enough that they grow indistinguishably into each other. When the piece is finished, no one would know how many plants went into making the hedge or where they were individually located. When building a wall of a house, one would never worry that all its individual parts -- 2 x 4's and drywall sheets -- must touch and can never be retrieved from the installation. It only matters how the finished wall presents itself. Here, you're developing a tall screen from trees. The presentation of the overall screen is far more important than any individual tree that goes into making it.

This is an instance where photos showing what you're trying to screen would be useful. Do you need screening at ground level, or screening views of second story neighbor's windows. Generally screening for privacy in-lieu of fences implies plant screening using shrubs more so than trees.

To answer your question about proper tree spacing, a design may have some trees planted close together, others at maximum spacing for mature sizes. How you do this depends on species being used, effects desired, how the trees in question adapt to closer spacing.

So either some plans or sketches or photos may get you better responses.

This is an instance where photos showing what you're trying to screen would be useful. Do you need screening at ground level, or screening views of second story neighbor's windows. Generally screening for privacy in-lieu of fences implies plant screening using shrubs more so than trees.

To answer your question about proper tree spacing, a design may have some trees planted close together, others at maximum spacing for mature sizes. How you do this depends on species being used, effects desired, how the trees in question adapt to closer spacing.

So either some plans or sketches or photos may get you better responses.