I dont quite understand how an 800 could be 80%. 800 is the highest score available. To my simple mind, that would make it 100th percentile.
An 800 is 80th percentile because 20% of the people that took the test got an 800.
The percentiles mean “you did better than xx% of the people who took this test”.
If you got the same score as others, then you didn’t “do better” than them, you did the same 
I think you would be spewing if you got an 800 with 0 mistakes, over the student who got 800 with say 4 mistakes.
When going through the trials, i said to myself that I should always even guess the ones Im not sure about. Its better to try questions that your unsure of rather than leave.
I have a little rule where if you do 50 questions, get 6 wrong, 44 right. The 6 wrong would be -0.25 marks each, bringing the raw score down to 42.50, which in my opinion is still good enough to get an 800.