Hmm, I think I'd take that a step further: your casting "level" is your Skill Rank. Knowledge is distinct from skill, so you may be highly skilled--Rank 20--but have limited knowledge--Rank 10--due to lack of a tutor, time to research, etc. Likewise, you may have Rank 50 knowledge (there's a Flaw for this, I think), but only low Skill--Rank 5.
If you have knowledge of it, you can overcast it past its level to get longer duration/range/whatever from the description, but that affects the level you're casting at (though not Power Point expenditure). Thus, a Skill Rank of 10 can cast the 10th level spell at 11th level, but only with a successful SCMR. Heck, the same caster--assuming he/she had the knowledge--could cast the 20th level spell on the list at 30th level, but only with a successful SCMR.
Options are good, so long as they have risks and costs.