It is the industry standard for installed sound. Any installer that is worth his merit has EASE.
Since EASE is a design tool that might be better stated as any designer rather than installer and you might also want to add other alternatives such as CATT-Acoustic and Odeon, both of which use Common Loudspeaker Format or .clf files. However, the usual cautions here that having such program and even using them is not necessarily the same as using them properly. These programs offer quite a bit of flexibility in how you can calculate and present the results, which is very useful when properly applied but that intentionally or unintentionally misapplied can give misleading results.
It is unfortunate how many times I've seen parties trying to sell a particular solution provide EASE predictions where the calculation and presentation parameters appear to have been manipulated to make their proposed solution look good. Whether that was intentional or the result of a lack of knowledge on the part of the person doing the work, neither is very reassuring.
Clark, to your original question, determining the on-axis overall level, or at least a conservative value for it, is pretty simple. However, that is not addressing aspects of the system performance such as frequency response, coverage and intelligibility. That also does not address practical factors related to both the system performance and installation. Or code compliance, Owner Standards compliance, etc. All of these, and possibly other factors, are elements in an typical installed system design and there is no single calculation to address them all.