CustomVanillaEnchants lets you configure value expressions for registered enchantments.

Run /cve list in console or in-game.

> cve list
/cve list <enchantment>
Available Enchantments: blast_protection, feather_falling, fire_aspect, fire_protection, knockback, projectile_protection, protection, sharpness

To view configurable entries of the knockback enchantment, run /cve list knockback.

> cve list knockback
Enchantment ("knockback") Configuration
damage_bonus: 0
force: level * 0.5

To change knockback enchantment's force value to level * 0.35, run /cve set knockback force level * 0.35.

> cve set knockback force level * 0.35
Updated configuration for knockback->force
Outdated Value: level * 0.5
Updated Value: level * 0.35

default can be used as a placeholder for the configuration's default value. For example, /cve set knockback force default * 2 sets knockback's force value to level * 0.5 * 2 (= level). To reset knockback enchantment's force to its default value, run /cve reset knockback force.

Changes take place right away, a server restart is not required. See the arithmexp wiki for a list of constants, functions, and operators that can be used in expressions.