As Doug said there is not a direct correlation between outside temp and box temp. The effects are direct temp gain, in other words heat leaking in to the box. 40 years ago the freezer would have 6 inch thick walls, now they want to maximize interior space, and other than the vacuum insulated models...
The other issue is the ability to condense liquid ammonia in the top condenser coils. At design pressures this stops at about 130 degrees, which is the upper limit, and is also the approxi,ate limit of compressor based refrigerators. As long as it can condense adequate liquid ammonia, the unit will cool, with the lower limit being around 40 below zero, the boiling temperature of the ammonia at the partial pressure of hydrogen.
You want to make them work well, install an ARP, optimize the venting and add as much insulation around it as you can.
