Calculation of percent inhibition requires dividing one number by another. Since both numbers have their own standard deviations, the standard deviation of the quotient is calculated using the appropriate propagation of error equation for division, which can be found here:

https://chem.libretexts.org/Core/Analytical_Chemistry/Quantifying_Nature/Significant_Digits/Propagation_of_Error

Mr. Shapiro, 

Thank you yet again, although I didn't find your link too helpful, I was able to find an explanation more understandable for my liking. 

Cheers again,

PD