How to get the attack damage value of a held weapon via procedure

The latest section of my project requires me to get the attack damage value of the weapon being held by the player.

Under certain circumstances, the weapon used by the player is supposed to multiply its own attack damage by 2, so I plan on doing this by directly getting the attack damage value of the held weapon and applying it to the target. I've figured out how to apply a flat damage amount to the target entity and how to find the item held in the player's main hand, but I am having trouble getting the held weapon's attack damage value. Is there a way to do this?