If i have worked out say a total heat load for a space to be 4kw for -20 c environment.

Do I select a coil and cu that is rated at 4kw evaporating pressure -20c from their printed charts.

I ask this because I was taught to always add another 10 degrees, ie: If I am evaporating at -20C then I read off -30C from the chart

All this stuff is mighty confusing. I need an education how to correctly select equipement