This is what TCS - 'Terminal Capacity Selection' is for, top page 3, then more detail explanation after network charging on p5. Runs from 0 to 100% for lipo, other batts may have higher range.

Basically after batt is set up if you want it to stop at 50%, enter 50% in TCS, charger stops at 50% capacity based on the setup you input. Want 80%, enter 80% in TCS. Couldn't be easier - name could be better though, its not real obvious what TCS is for.