Simplest, that is, from the perspective of "How many spells can I cast at Level X?" There is a way to maybe make it simpler, by looking at it from a different perspective: "At what level can I add another spell of Level Y to the number I can cast?" It's a matter of whether you look up your class level first or the spell level first; the first method, when put in table form, requires reading one line, while the second method in table form would have one line per spell level, something like this:
|# of spells|
|Spell Level||per level:||1||2||3||4|
Or, putting it into a shorter, formula format:
Magic-Users can prepare and cast two spells of a given spell level when their caster level is double the spell level, or one spell of double the spell level - 1. They add a third and fourth spell at:
- 3rd and 4th level, for 1st level spells;
- every three levels after (2 x spell level), for 2nd through 4th level spells;
- every two levels after (2 x spell level), for 5th level spells;
- every level after 12th, for 6th level spells.
Edit: I wrote "half" instead of "double" in the "shorter formula" above, even though the following bullet points make it clear I'm talking about doubling. Fixed now.