FS_BATT_CUR_RTL. A value of 22 gets you on the ground with about 10% remaining. A higher value will RTH sooner leaving a higher % remaining.
/usr/bin/extSettings.conf. The defaults are as follows:
RTL_LOIT_TIME. It is in milliseconds. It is the amount of time the copter will hover prior landing. If simply turn it up to several minutes or whatever suites you needs, you will have the same effect.