The power level of spell is never clearly defined, the level increasement of modified spells is not defined (effects of different dices, durations ect. ect.)
In this the Metamagic feats help a little, though.
If you create complitely NEW spell, the Book tells you to compare the general power and effectiveness of the spell to existing ones to determine the Spell level.
I'd call that "Guesswork".
<{POST_SNAPBACK}>
I win :D