There is a great VRD for RF planning located here:
http://www.arubanetworks.com/wp-content/uploads/Indoor80211n_2012-05-31.pdf
In there, please note the considerations of the deployment model you wish to deploy. There are things like client density, signal strength requirements, transmit power of the client NIC, etc... It's not just about how far an AP can reach...it's also about the AP hearing the return traffic from the client.
So...here's an excerpt which may help:
...since approximately 2008, most new enterprise wireless LANs have been deployed using a “capacity model” approach. In this case, we use many more APs because our concern is ensuring that a minimum speed is available throughout the coverage area.
Today, data and voice designs use a capacity approach. The primary difference between them is the choice of minimum data rate. Data-only deployments have less rigorous needs than voice deployments. For a full explanation of the data rates available see the Aruba 802.11n Networks VRD.
Capacity Model (High Bandwidth)
The capacity model is for dense deployments with high device counts and traffic rates. A capacity-based deployment might consist of APs placed roughly 45 to 60 feet (approx. 13.75 to 18.25 meters) apart running at 25-50% or 50-75% of power. In general, the transmit power of the AP should be set to match that of the least-capable device in the network. If the requirement is a “desk-top like” experience for employee laptops, where the employee can run multiple applications simultaneously, the site requires a capacity-based deployment. Aruba recommends capacity-based deployment for all office and education settings.
Coverage Model (Low Bandwidth)
The coverage model is for low-bandwidth deployments, coverage is required for applications such as a scanner solution or limited guest access. A coverage-based deployment might consist of APs placed roughly 70 to as much as 200 feet (approx. 30.5 to 61 meters) apart in an open space, running at 50-75% of power. If redundancy is not required, APs can run at 100% power, but this typically is not recommended. As an example, if the determined application is a scanning solution with minimal traffic, the site might be a good candidate for a coverage-based model. This deployment would consist of an AP installation base with clients that associate at greater distances and at lower traffic rates. This coverage model would mandate a ceiling deployment.
Aruba no longer recommends coverage-based deployments as networks that had very few clients previously are now seeing new services deployed.