Minus 8 wasn't your score.
18 par 5 holes have a par of 90, so your score was 82, not -8.
On a par 72 course -8 nets a score of 64.
On a par 71 course, -8 nets a score of 63.
On a par 70 course, -8 nets a score of 62.
The par 5 and par 3 courses (par fof 54 for 18 holes) don't count towards your scoring average because of their par is to far out of line from what is normal - pars of 72, 71, or 70.