You don't say where you live, but in North Alabama we run 25% max.
Add one gallon of anti-freeze and then fill the system with distilled water. It will be 24% to 30%.
If you drive your Cobra in 32 degree weather you may need 50%
antifreeze. But remember distilled water is 20% more efficient in cooling your motor than 50% anti-freeze.
Most of us keep our Cobra in the garage during cold weather.
Dwight