|Ability Score||Max. Spell Level||Bonus|
|1-8||NA||Can't Cast Spells|
Maximum Spell Level is quite simply the highest level spell a character with that ability score can cast. I recommend allowing characters a bonus +1 to an ability score of their choice every 4 or 5 levels (I go with 5), so a character with a spellcasting ability score of less than 16 has ample opportunity to rectify that before his experience level opens him up to high level spells.
Bonus is pretty easy. A character with high ability score gets a bonus of 1-3. They may cast extra spells each day of a total spell level equal to that bonus. So a character with a score of 17 in his spellcasting ability gets a bonus of 2, meaning he can cast 2 extra 1st level spells or one extra 2nd level spell. The character's experience level must still qualify him for the bonus spells, in other words, a 2nd level character can't cast a third level spell, even with an 18 in the relevant ability score. If you use the optional Cantrips or Orizons (zero level spells) for Classic D&D rules that are floating around on the internet (as I do), count each 'zero level' spell as 1/3 of a spell for this bonus. A 1st level magic user with a 14 INT would get a bonus of 1, and could either cast one extra 1st level spell, or 3 extra cantrips each day, for example.
As a house rule, I do not grant the Bonus spells to Elves, I feel that their combat prowess more than balances having a couple fewer spells per day, and the extra training required to learn martial skills in addition to spell casting is more than enough "in character" justification for the restriction.