You have lost 18% body fat. You cannot subtract percentages like that. What you need to do instead is calculate the percentage of body fat lost. You've had 104 lbs BF before. You now have 85 lbs BF, which means you lost 19 lbs. 19/104 = 0.18, or in other words you've lost 18% of the body fat you've had before.

Think of it like that - if a party has 100 votes, and it converts to 1% support. If they instead got 200 votes, they would have 2% support. But that doesn't mean the number of people that support them increased only by 1%. It doubled, or increased by 100%. That's the difference between increasing by x percent and by x percentage points. https://en.wikipedia.org/wiki/Percentage_point