Those are 7th and 6th level spells... the 1st level spell can do the the same 50% of the times. And that's a lot for the party, as it saves a 5&6 or a 7th level cleric spell, 50% of the times, in close combat when the mage needs just a few seconds more so the melee'ers can finish the monster...
So what is it that you're suggesting anyway? That the low level spells should be useless compared to the high level ones or what? If 50% is too good for you, what percentage do you prefer? 20, 10%? Gimme a break, no one would use your "nerfed" low level spells ever.