Mega Man X5 ranking system documentation by Akiteru
Done on USA version

POINTS TIERS

STAGE TIME:
less than 1'00"00 - 8 points
less than 2'00"00 - 4 points
less than 5'00"00 - 2 points
5'00"00 or greater - 1 point

ENEMIES DEFEATED:
less than 40 - 8 points
less than 80 - 4 points
less than 150 - 2 points
150 or greater - 1 point

DAMAGE RECEIVED:
less than 16 - 8 points
less than 32 - 4 points
less than 64 - 2 points
64 or greater - 1 point

MAX DAMAGE DEALT TO ANY ENEMY IN THE STAGE (INCLUDING BOSS):
8 or greater - 8 points
6 or greater - 4 points
4 or greater - 2 points
1 or greater - 1 point

NUMBER OF TIMES GONE INTO "VIRUS MODE" DURING THE STAGE:
0 - 8 points
1 - 4 points
2 - 2 points
3 or greater - 1 point

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NEW RANK CALCULATION

MEH:
less than 32: decrease rank
32 or greater: same rank

PA:
less than 32: decrease rank
less than 40: same rank
40 or greater: increase rank

GA:
less than 24: decrease rank
less than 36: same rank
36 or greater: increase rank

SA:
less than 20: decrease rank
less than 32: same rank
32 or greater: increase rank

A:
less than 12: decrease rank
less than 32: same rank
32 or higher: increase rank

B:
less than 15: decrease rank
less than 28: same rank
28 or higher: increase rank

C:
less than 12: decrease rank
less than 20: same rank
20 or higher: increase rank

E:
less than 16: same rank
16 or greater: increase rank