The outdoor design temperatures are not the absolute highs and lows. They are supposed to represent the typical highs and lows you may see in the summer and winter.

Thanks for clarification, now I know the high temp is right and to adjust the "low".

I'll be back after getting more detailed estimates and questions answered.

You do not size for outdoor temperature extremes both heating and cooling nor for extra people in your home above the usual number that normally live there. You can use a fudge factor though and make a comparison to typical to see how this might affect sizing.

IMO