How cold your current coolant will protect to, and how much to drain and replace with straight antifreeze to reach the mix you want.
You drain some of the existing mix and replace it with 100% concentrate. The formula finds the amount that lands exactly on your target. In practice, an engine block doesn't drain completely from the radiator petcock, so test again after the engine has run and mixed the coolant.
Freeze points are approximate values for ethylene glycol, the most common antifreeze: about −34 °F at 50% and −62 °F at 60%. Propylene glycol coolant protects a little differently, so check its label. Use distilled water for mixing to avoid mineral deposits.