The purpose of the antenna gain setting in the radio is to tell the radio what the gain of the antenna is to prevent you (or the radio) from "accidentally" exceeding legal EIRP for whatever regulatory domain you told it you are in. It also allows the CPE's to auto adjust TX power and prevent them from exceeding legal EIRP.
If an AP is configured to auto select frequencies then it uses whatever you told it the antenna gain was in order to know how much TX power it can use whatever frequency it picks.
If you are setting TX power to max and then adjusting the antenna gain setting then you are doing it backwards and if the antennas you are using actually have more gain than whatever you set it to in the radio you are probably exceeding legal EIRP.
So that setting isn't really a setting as it is more for informing the radio what gain antenna it is connected to.
The range setting is not about power it's Sync / Timing. Any AP's you have configured to use Sync must all be set to the same max range setting. A shorter range also has some small performance benifit but I'm not sure how noticeable it might be.
The only relationship distance/power really have to each other is you might turn power down if it is a really short distance and you are trying to prevent self interference (that is also the reason for the Max CPE RSSI setting, it makes the CPEs use no more power than they need to so they aren't blasting RF miles farther than they need to).