The best way to see if you need to increase or decrease power is to do a quick survey of your environment.
First evaluate to see what kinds of clients you have and what do they like. As an example - with iOS devices I evaluated RSSI signal levels all throughout my house and wanted devices to roam about halfway thru the house. I ideally placed my APs high & unobstructed. I found a power level of 23 (United States) gave me two nicely defined -70 dBm 5ghz cells. Apple devices like to roam at that threshold and with r/v/k I get predictable fast roaming. I just have smart IoT devices on 2.4 ghz so I just made sure that the signal got to my smart devices (no roaming needed).