Workshops Are The Safest of Places With Junk Shield Perk

The perk is affected not by junk weight, but junk quantity carried.
A base Junk Shield perk while carrying no junk will not give any damage resistance. Carrying 1 to 5 pieces of junk will give 1 point of damage resistance, carrying 6 to 10 will give 2 points, 11 to 15 will give 3, and so on until the max resistance of 10 points at 46 or more pieces of junk carried.
The damage resistance will be additionally multiplied based on the level of the perk; level 1 will be a 1x multiplier, while level 2 & level 3 would be 2x and 3x multipliers, respectively.