Headshot Damage Multiplier question

So let’s assume a weapon does 10 damage, and it has a 150% Headshot Damage Multiplier.

Are we assuming that 100% damage multiplier is the base damage? So 150% means 1.5 times the damage (in the example above being 15 damage for headshots).

Or is a 100% damage multiplier already doubling the damage? So for the example above, 100% would mean 20 headshot damage and 150% would mean 25 headshot damage?

Am I making sense? I’m a bit stoned and math is not my strong suit.