So there are a couple of tools that you can use for planning and setting a base line. One is ePMP Capacity Planner Tool R2.4.3 paired with the ePMP Capacity Planner User Guide. The second tool is LinkPlanner.
These tools allow you to input your AP and clients data (either proposed or existing) and will give you some suggestions as to what capacity and throughput you should be seeing.
Once you've physically deployed your radios, you'll want to pay attention to a few things... starting with the AP check out:
Monitor -> Performance -> System Statistics (lots of session drops usually indicate heavy interference issues)
Monitor -> Performance -> Downlink Packets Per MCS, and Uplink Packets Per MCS (you want the majority of your traffic using higher MCS levels). If you find that most of your data is flowing through lower MCS levels, this will cause AP frame utilization to go up, and clients will have difficulty getting their QoS BW allotment along with an increase in latency, jitter, or even packet loss.
Monitor -> Performance -> Downlink Frame Time, high values (up to 100%) indicate that your AP cannot transfer more data and you may start to see increased latency, jitter, and even packet loss to some or all of the clients on the AP. An AP with lots of poor connections with poor modulation will result in high frame utilization with low data transfer rates during peak usage periods. An AP with lots of good connections and high modulation will results in a high frame utilization with high data transfer rates during peak usage periods.
Monitor -> Wireless, use this area to get a quick feel for the overall health of the AP. Ideally you want to be seeing a lot of blue and green, indicating good client modulation. If you're seeing lots of yellow and red, then that's a sign that you're either experiencing interference or you have low signal or SnR quality to clients.
Monitor -> Throughput Chart, during peak hours, take a look at this chart to look at overall/aggregate AP throughput, then compare this to the frame utilization. If there's not a lot of data flowing, but you're seeing high frame utilization, this indicates that you have client(s) that are having issues. Try to identify and improve these clients to increase overall throughput and reduce AP frame utilization.
Tools -> Wireless Link Test, use this tool to find out if your clients are getting the proper BW that you assigned in your QoS rules, you can also use it to help find bad clients that aren't getting the BW they're paying for and may also be tying up valuable AP frame utilization.