I have decided to make changes for this rule in the player’s handbook.

On page 10, left hand column, there is a chart I am modifying for my game world.

Int 15-16 70%, 14 to 22
Int 17 80% 16 to 24
Int 18 90% 18 to 36
Int 19 97% 20 to all

These are spell book amounts. The spells per day is a separate chart in this blog.