Different opinions..
I have heard the parry is 50%Act before, but I find conflicting rules that that don't backed it up and just plain don't like it.
I feel the problem lies with OB being allowed to apply vs missile DB.
If it read, "Melee OB can't be applied vs Missile attacks. Only shield bonuses, Qu bonus, etc can be applied vs Missiles and this "Parry vs Missile" is a 50%Act." It would make a lot more sense. To me..
jaranka, I think you are correct.
"Note: Even if a combatant parries with 100% of his
OB, he must still make a +0 OB attack—he may still
fumble or perhaps roll high open-ended." pg 92 RMSS.
An attack requires 100 to 60%Act; -1OB/1%Act less than 100.
"Stun appears to be a special case that allows parrying, albeit with limitations, while only having 50% activity."
Yeah, it does muddy the waters OR reinforces the 50%Act parry opinion. Which ever way you go on the topic.
If you compare Calm Song to 4th lvl Calm, Ment Base, it could be similar. "Target will take no offensive action, and he will fight only in self-defense."
Fighting in self-defense sounds like a "full parry", all OB on DB, +0 attack roll, to me.
IMHO, Calm Song is 1st lvl (easier to access) because the List is Controlling Songs. But I handle them both the same.