You don't really need to modify the voltages; the defaults should work.
The one thing you may need to adjust is the input current limit. If you find that charging starts, then stops a second or two later, then repeats, you need to lower the maximum current. The reason is that if the panel is small, too much current is drawn during charging, which causes the voltage to drop, which causes charging to stop, so the voltage rises again, and the process repeats.
This video is for the Tracker One, but it also mostly applies to the Boron, which has the same PMIC. The main difference is that the absolute maximum voltage on the Boron VUSB is 17V and it's much higher on the Tracker One. Charging the TrackerONE with a solar panel