[Radek]

Compare characters with SPI 8, 10, and 20.

If the first character gets mana points during level up, he gets only 8 mana points. At level 50, the "reasonable value" will be 580 but even if we suppose that the character gets mana points every time he levels up, the character will have only 420 mana points.

The second character can get 10 mana points every time he levels up. That means, his number of mana poins is able to keep in step with the formula for "reasonable value". At level 50, his "reasonable value" will be 600 and he can get his 600 mana points theoretically.

The last character gets 20 mana points. He can expect that he gets mana points every second level up. He can be sure that he will have his "reasonable value" at level 50, which is 700.

Summary:
The first character suffers from low SPI. The value below 10 causes that he cannot get the mana points, which he should have.
The second character has a theoretical chance that he will get his "lawful" amount of mana. Well, it's only a theory and he needs a bit better SPI.
The last character need not fear that he won't get his mana points.

It's the multiplier '10' which makes SPI 10 a "breakpoint".