Looking at the Perdo Vim guidlines on Page 160
So does this work out thus?
Level: 20
(Base Effect +2 Voice) --not sure about this either.
Reduce targets casting total by 20.
?
Almost: the reduction would only be 10, since you need to halve the final level. The 2 extra magnitudes built in the guidelines are indeed there to compensate for the Voice range.
No.
I'll use an example from the book Guardians of the Forest if you don't mind?
According to this the Base effect is then 40 (55-5-10) seeing as it reduces casting totals by 25, and as two times 25 is 50, and 50 is two magnitudes more than 40, you example would reduce casting totals by 15. (its Base effect is 10, and half of ten is 5, and two magnitudes more than five is 15)
Here's my take;
To Neutralize the Annoying Apprentice, PeVi 25
(Base , Range: Voice +2, Duration: Diameter +1, Target: Individual)
Decreases all Casting Totals made by the target by 15
Very nifty effects... Especially in items:
Room of Endings
This small, metal box is buried far, far underground and is filled, almost to the very top, with water. It is also enchanted with two powerful Vim effects.
Crawling Despair, PeVi 65
(Base 50, +1 Touch, +2 Sun)
Under this effect, all casting totals are decreased by 35. With a Penetration of 50 (+25), two uses a day (+1) and an environmental trigger (+3) its final level comes to 84
Inevitable Doom, ReVi 30
(Base 15, +1 Touch, +2 Sun)
This effect hinders anyone inside the box to use any spell of level 25 and below. With a Penetration of 50 (+25), two uses a day (+1) and an environmental trigger (+3) its final level comes to 59
Sounds nasty enough? I'm sure gonna want to implement this thing. Especially combined with a teleport circle.... 
My understanding is that the two magnitudes get added before division, at the same time as R/D/T/whatever adjustments are taken into account. This is consistent with how you decomposed Hinder The Gift.
Urien's level 20 spell has a base of 10 due to Voice range. Add two magnitudes and we're back to 20, divide by two and we have a 10 point reduction.
d31m0z the Mad's Hermetic Deathtraps, Volume 1. 
I certainly uderstand your point there, but if you have the Guardians of the Forest book, there is an example on pg. 60, middle column, third paragraph.
I luv' them deathtraps! Hafta make some more, eh?
Great! An example!
Okay..this is how it should be (remembers higher math skills)
Do the things inside the brackets first, then do the stuff that goes with it..
Hinders a spell = 1/2 * (Level +10)
[10 being two magnitudes in most cases....]
In the example provided...
69-14 (Penetration, uses per day, trigger)= 55
55- 15 (touch and trigger)=40
40 being the final level of the spell.
Plugging it in yields...
1/2*(40+10)
...1/2*(50)
......25
Thanks d31m0z!
I strive to please! 
(Woot! Journeyman!! GO ME!)
And we are in agreement about that. However I maintain that you have analyzed Urien's original spell incorrectly, dividing before adding the two magnitudes. Done this way, Hinder The Gift would provide a 30 point reduction, not 25.
Congratulations, Journeyman. Now you need to work on your masterpiece.
...
His original spell was first of all not complete; it had a Base Effect of 10 and +2 for Voice Range for a level 20. But then it'd have a Momentary Duration. Sort of ... useless, eh?
Anyway, let's do the formula:
Reduces Targets Casting Total by (Base Level + 10 Levels) / 2
Then we calculate: (inside the brackets first) (10 + 10 = 20) / 2 = 10
His suggested spell would target an individual Momentarily at Voice Range and reduce said individuals Casting Total by 10.
We apply the same formula to Hinder the Gift:
(Base Level + 10 Levels) / 2 = Reduction in Casting Total
(40 + 10 =50) / 2 = 25
So we can then see that I have my foot in my mouth, while calmly relaxing in a nice, cool salad... For some reason I managed to confuse myself totally, and start talking past you there mate.
'twas fun though, ey? 
Yeah my example was simplified...I was trying to figure out the equation...not the modifiers due to range and duration...
d31m0z:
I'm glad I could provide you with an opportunity for such a healthly AND tasty meal...
That can be sufficient as a fast-cast counterspell. Formulaic PeVi with fast-cast mastery should be part of every hoplite's arsenal.
I made a character with such a spell at Touch range and Diameter duration (penetration is key). She then uses short-range teleportation to catch her target.
Well, 2 magnitudes ain't exactly the same thing as 10 levels, but I think I've picked on you enough already.
What kind of dressing on your salad?
Yup. 
I really, really want to harm you now... 8) Something to do with squirrels, and their favorite food; nuts! Conjuration of Squirrely Doom! Multicast! 
A light mustard and pepper dressing... Very tasty (ask me for a recipy)