I will give you that the general text of the book is VERY unspecific with this. I was never sure for the longest time, and maybe this isn't even a really for sure... Maybe the idea of what they wanted got scrambled somewhere.. But anywhere, off to citing...
pg 411,
Decide how many shifts of power you want
to put into the spell. Take 1 mental stress,
plus 1 for each point of power greater than
your Conviction modified by any power
bonuses from a focus item.
pg 250,
Decide how many shifts of power you want
to put into the spell. You take mental stress
for calling up power—the minimum is one
point of mental stress. The cost increases
if you reach for power greater than your
Conviction, inflicting additional mental
stress equal to the difference (so a spell with
power three higher than your Conviction
would inflict 4 points of mental stress).
pg, 256
Every shift of power you summon past your
Conviction costs one additional point of mental
stress during casting; so a spell with a power
rating three higher than your Conviction would
inflict four points of mental stress when cast
(one for summoning up to Conviction, plus one
for each additional step on the ladder). Just like
any other type of stress, this could force your
wizard to take consequences in order to keep
himself upright.
Basically, in all these examples, and I could look for maybe a few more, they refer to the mental stress in Points, rather than a 1, 2, 3, or 4 ect, stress hit, which is how they refer to taking stress through out the rest of the book. It also never goes into bigger number examples using a higher mental stress hit. All examples of going higher, are referred to using a consequence to help take off some of the stress. Sorry >.<
And here is a citation of backlash, with a different wording, using stress hits like everywhere else in the book, rather than points of mental stress...
pg, 256
There are basically two kinds of trouble:
backlash and fallout. Backlash affects the
wizard; fallout affects the environment or other
nearby targets. Like everything else, backlash
and fallout are measured in shifts—in this case,
the difference between your failed Discipline roll
and the gathered power. The worse the failure,
the more the spell’s energy goes haywire, and the
worse the effects get.
You get to choose how much backlash you
absorb, with the rest going to the GM as fallout.
Particularly self-sacrificing wizards may choose
to take some or all the excess as backlash—especially
if there are allies nearby who might bear
the brunt of the fallout of the failed spell.
Any uncontrolled power taken as backlash
remains a part of the spell and does not reduce
its effect. Fallout is different: every shift of
fallout reduces the effect of the spell.
Backlash means that the spell energies run
through the wizard, causing injury or other
problems. This manifests as shifts of stress,
which could require the taking of consequences
in the usual way. Fortunately, you can choose to
take the backlash as either physical or mental
stress (but not split between both), which means
your wizard can keep his mental stress track
open for more spellcasting if he needs to.
Example: Harry must cast a spell with a
Superb (+5) control target without using
incantations or focus items. Without these
benefits, he fails the Discipline roll by 5. He
doesn’t want to hurt anyone around him or
cause any unintended environmental effects,
and he needs the spell to succeed in full, so he
chooses to take a 5-stress physical backlash. He’s
already taken some physical stress during this
scene, so he ends up having to take a moderate
consequence of Utterly Exhausted in
addition to a 1-stress physical hit. Ouch. But at
least the spell is still cast at full power.
Its a bigger example, but I didn't want to leave anything possibly important out, hehe >.< I know this won't "help" persay, but it should hopefully clear a few things up..