IAPs have an ARM minimum and maximum transmit power that you configure. That is the range of transmit power that APs will be allowed to have:
http://www.arubanetworks.com/techdocs/Instant_41_WebHelp/InstantWebHelp.htm#UG_files/ARM/Configuring_ARM-Features.htm?Highlight=arm minimum
You should make the difference between the minimum and maximum no more than 6 so that the difference between the highest possible AP transmit power and lowest possible transmit power is only 6. This is so that the performance of clients is not uneven based on which access points they connect to.
The maximum transmit power of the access points should not surpass the transmit power of typical devices. If an access point transmit power is significantly higher than the transmit power of the client that attaches to it, the client will think it can communicate with an access point that can no longer hear it well, creating a "sticky client" issue. This will create application performance issues on the client, as a result. Typically you can start with a minimum of 12 and a maximum of 18. A much more technical explanation of this is here: http://www.greatwhitewifi.com/2016/05/14/power-matching-transmit-power-or-eirp/
The document I referred you to does make references specifically about controller-based parameters, but the ARM minimum and ARM maximum transmit power is the same parameter between controller-based and Instant Cluster-based deployments.