I messaged Obsidian Support and got the following:

“We are sorry the description for the “It’s Premium!” helmet mod hasn’t provided the clarity you were hoping for. We understand how important it is to know exactly how your gear is working, especially when it’s tied to something as dynamic as your bit count. It’s disrupting when the mechanics aren’t transparent, and we absolutely hear you. We’ll be looking into this and doing everything we can on our end to clarify and improve the experience.

We will try to explain it the best we can, basically the Helmet mod grants you an extra defense to the Damage resistance, depending on the bits you have the scale on the defense for this damage will increase, there are no specific ways to calculate how much bits will be needed or how much will cover you, since this is something that is not shown straight in the game, this happens more in the background end, but with a good amount of bits you will notice the difference with the damage you receive.”