How to get the reverse percentage (not amount of reverse percentage)?
Just use percentage of increase/decrease.
$$\frac{b-a}{a}$$
Where $a$ is the initial value and $b$ is the final value.
$$\frac{100-117}{117} = -\frac{17}{117} \approx -0.14523$$
Multiplying the value by $100$% gives the percentage. So, subtracting $14.523$% of $117$ from $117$ will give $100$.
Just remember that when $b > a$, the answer will be positive, so there is a percentage increase (must be added), but when $b < a$, the answer will be negative, so there is a percentage increase (which must be subtracted).