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To raise concentration you must drain the same volume you add, since the system is already full. Glycol at 50% gives about -37 degrees of freeze protection and raises the boiling point, while going much above 60% actually worsens heat transfer.
Coolant mix
glycol deficit = capacity x (target - current); drain and replace that volume with concentrate
Pure glycol has poor specific heat and higher viscosity, so it cools worse than a 50/50 mix and freezes at a higher temperature than 60% glycol.
Around 60-70% glycol reaches about -55 degrees. Beyond that the freeze point rises again, which is why pure antifreeze freezes at only -13.