As general information to cover a couple of your points, keep in mind that antennas and environment will be the primary factors in how far a signal goes at a specific power level.
Formulas are nice for ballpark, but you'd be surprised how many things can screw up a "straight forward" installation.
Any objects intermediate to the AP and client can have a very broad spectrum of effects. A tall shelf can cause multipath, a human can suck up a big chunk of signal, leaded glass decorative partitions can create a shadow such that a client won't connect from 20 feet away.
While it is a Good Thing to have paper plans and calculations, they should be confirmed with a comprehensive site survey.
Once you have a good idea of how reactive (or not) the environment is, you can match up the desired operational areas with antennas that can create the pattern and the power necessary to meet the goals.
You may also be fighting noise / interference and some adjustments to your preliminary power levels maybe needed.
The best way to reliably judge is to perform a survey; initially for design, then post-installation to pick up nulls, dead spots, hot spots, and places where the signal goes beyond the intended area.
Corrections can be made by using different antennas, changing power levels, moving the AP/Antenna(s), moving objects in the room for better signal path, etc.
A final survey for signoff should (IMO) be done, so the customer has an opportunity to see that the goals are met, everything works as agreed, all issues have been resolved, AND THE DOCUMENTATION MATCHES THE "AS BUILT" configuration (so payment will be forthcoming and unhindered).
"No job is complete until the paperwork is done"
