The Ranking system I use is a combination of the Fate versus ranking system and that of other RPG mangas and light novels I've read, it's relatively simple to understand, the actual ranks aren't used in-universe, it's an out of universe thing I use to show the difference in abilities and to show how powerful an ability may be.
The Rankings go as follows:
E-
E
E+
E++
E+++
D-
D
D+
D++
D+++
C-
C
C+
C++
C+++
B-
B
B+
B++
B+++
A-
A
A+
A++
A+++
S-
S
S+
S++
S+++
SS-
SS
SS+
SS++
SS+++
SSS-
SSS
SSS+
SSS++
SSS+++
EX
The minus symbols mean that every ability or skill with it is overall 50% weaker than the same rank without the minus and up to 200% weaker than the same rank with three-plus markings.
A single plus marking is 50% stronger than the same rank without it, a second plus becomes 100% and the final plus shoots it up to 150%
In certain situations a Rank with a Plus symbol is capable of temporarily rising by one rank, so E+ could turn to D. This usually only happens during life or death situations akin to how Humans can pick up things that would normally be impossible to pick up when trying to save their child or themselves.
Each rank has a number Value to it going through each rank from - to +++:
E- through to E+++ Goes:
5
10
15
20
25
D:
30
35
40
45
50
C:
55
60
65
70
75
B:
80
85
90
95
100
A:
105
110
115
120
125
S:
130
135
140
145
150
SS:
155
160
165
170
175
SSS:
180
185
190
195
200
Despite the numerical values only being 5 apart from one another you can see that lower ranks are exponentially weaker than those of higher ranks, this numerical value is used in-universe but only by select places that require measurement of magical or physical power such as Dungeon Exploration Guilds or Adventurer Guilds.
EX rank does not have a numerical value. EX is the exception, hence why it is called EX
SSS can't be compared to EX as EX is the exception to the ranking system, a level of power that Humans don't have the capacity to measure, it's not infinite just immeasurable.
EX rank skills are the strongest they can possibly be and anyone with any rank below SSS can't compare to those with an EX rank ability.
Only SSS rank or above can compete with EX rank capabilities and even then depending on the person and situation they may still be heavily outmatched.