Is this typing speed calculation justified?

[Techieey]

I just gave a local government typing exam and wasn't qualified.
The test was of 10 minutes duration.
They used the following formula:
Net wpm = Gross wpm - Errors
So, my gross wpm was 45 and I made 13 mistakes.
According to their formula, my net wpm = 45 - 13 = 32.
The required minimum speed was 35. So, I didn't qualify!

Is their formula justified?
Please, suggest what should I do now? How to convince an authority that "your typing speed calculation is wrong"?
Also, share your thoughts if the authority can justify its formula.

PS: I was actually penalized 10 words for each error by their formula. Think, it would be 15 words for each mistake if the test had been of 15 minutes duration.

[toddhicks209]

The best suggestion I can offer is to tell the test providers about it and ask for a redo then practice a lot regularly.