The simplest answer is "where most people can see it comfortably".

Care should be taken to position the screen so that the maximum number of people can see it without needing to strain their neck in order to do so.
If you have troublesome sight lines, for example rows of pillars, you might be better considering two screens instead of one. The downside is the increase in cost but you should be able to use two smaller screens than one big one.

We always visit a site before a screen installation to agree the best place to combine visibility with aesthetics. To get the best use out of your screen while keeping it in touch with the design of the building itself.