A characters potency is what is displayed on their character stats and any in battle changes. There’s no min starting point.
To be clear, if the trigger is met for a debuff to be applied, then it will be applied unless the character evades the attack or resists the debuff. This could be where people think there’s a base 100% potency. L
Potency is subtracted from tenacity to get a character’s chance to resist the debuff. So 100% tenacity minus 50% potency is a 50% chance to resist.
Keep in mind, Tenacity has a min value of 15%. So if your potency reduces their tenacity below 15% then they get the minimum 15% chance to resist. The only way to remove that 15% base chance is with an unresistable debuff.