**Your calculation is correct, but you're misinterpreting the result.**
Let's have a simple example:
* Standard hourly pay (R) is $1
* Standard working time (S) is 10 hours.
* Actual time taken (T) is 8 hours.
Compared to the standard wage of $10 you're now earning -(10-8)^2/10 = -0.4. So you're earning $9.6 instead of $10, but **you worked 2 hours less!** If you keep working the same duration as your colleagues, you will earn more than them.