Minimum length of thermal insulator?

Looking at drawings of the thermal insulator, is 20mm the absolute minimum insulation distance on an extruder?

Is there a way to calculate the expected temprature diffrence. I.E if this length was to be 15mm instead of 20mm or 10mm?

Is anyone using an insulator with less than 20mm of insulation?

The 20mm dimention is based on this drawaing (35mm - 15mm = 20mm) from the hot extruder to the end of the insulator.

Maybe the length of insulator can be diffrent for a PLA Extruder to that of ABS Extruder ?

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