Shell Programming and Scripting

It's easier to comprehend if you try your formula with easy numbers.

A reduction from $400 to $0 would be -100%. Or all the money.
echo "scale=2;(((0 - 400) * 100)/400)"|bc
-100.00

A reduction from $400 to $100 would be -75%. Or three-quarters of the money.
echo "scale=2;(((100 - 400) * 100)/400)"|bc
-75.00

A reduction from $400 to $200 would be -50%. Or one half of the money.
  echo "scale=2;(((200 - 400) * 100)/400)"|bc
-50.00

A reduction from $400 to $300 would be -25%. Or one quarter of the money.
echo "scale=2;(((300 - 400) * 100)/400)"|bc
-25.00