The correct coolant ratio is 50/50 coolant/
DISTILLED water for maximum temperature effectiveness.
For future ref, if all you needed was to top off, you could just use DISTILLED water. The cooling system holds ~ 5 gals (20 qts) so even adding a qt or two won't throw off the mix balance much. Likewise, you could just add coolant if only a quart or two is needed.
Good luck