I use a modified version of this rule.
Instead of:
(Force / 2) ± spell's mod + net hits added
We use:
((Force + net hits added) / 2) ± spell's mod
It basically modifies the spell as though you cast the spell at a higher Force, but it does not change the drain type from stun to physical if the result is above your magic score. The caster can choose to add net hits to the damage in this way or may opt not to.
I think this little tweak helps in a few ways.
1) It helps discourage overcasting since you can still achieve high levels of damage if you hit well, whereas the RAW ENCOURAGES overcasting because adding damage from net hits is a 1:1 ratio to drain and increasing force is a 2:1. Just more efficient to up the Force. Sure, with the RAW ruling you'll get hit with physical when overcasting but it will be a smaller number to soak.
With the change I use it's the same amount of drain as if you overcast to a Force 8 and added no hits or if you cast normally at Force 5 and added 3 hits. The only difference is the drain type, and I see the higher damage with a stun drain as a reward for hitting well.
2) It doesn't penalize the player nearly so badly for rolling well.
3) It still helps to reduce the power of direct spells.
Interesting variant, but I'm confused about its use.
1) It's a valid compromise to add net hits before dividing, I can see how that's a happy Medium. I disagree on the need for Overcasting though....
2) Who get's penalised? The mage has the option to utilise as many net hits as they think they can handle (My most painful memory was chucking in 6 net hits on an Edge-assisted Stunbolt just because the BBEG HAD to go down, or we would all die. The pain was worth it in the end, and very cinematic
3) as 1) it's a nice medium between the two. Very cool.
My current GM is ambivalent on overcasting (as am I in the GM hat). I don't see any need to encourage/discourage it's utilisation. It's just an option to exceed your limits if required....
On #1:
Well, no NEED. Just that the existing rule
encourages overcasting.
Goal: To get 10DV to take out the guy before he can sound an alarm.
For this example we'll assume a magic of 6 and use Manabolt. We'll leave out dice pools since they the spellcasting pool would be the same each time. Instead we'll assume 4 net hits in both examples. We'll also assume 4 hit's on the soak test.
Optional rule as RAW in use:
Cast at Force 6, get 4 net hits, use 4 to get our target 10DV. The drain will be 7S. After soak it's 3S the mage receives to his/her track.
Cast at Force 10, get 4 net hits, use none to get 10DV. The drain will be 5P. After soak it's 1P the mage receives to his/her track.
Same DV to the enemy but the number of times you can cast the F10 one is 9 times with a full track (assuming low body score). But that mage can only cast the F6 spell like this 3-4 times, assuming a high willpower, or choose to use it less at a lower DV to get the number of uses back to what overcasting can do.
Additionally, the lower amount of damage taken will mean less wound modifiers to take dice away from future tests. With our example the mage takes 1/3 of a wound modifier for overcasting and 1 full wound modifier for not.
While not the worst thing in the world, this bothers me a bit both mechanically and thematically.
Mechanically it encourages overcasting because a combat mage is likely going to want to use the more damaging option that they can also use more often. This can easily be one of the reasons that people feel mages are unbalanced in combat. There are plenty of other factors that effect this, as it is a decision on the player's part, but many players will go this route.
Thematically, I don't just don't like the idea that overcasting is something that happens
all the time, and mages looking to deal damage fast will often use the overcasting option with frequency. Also, I don't like the idea that a mage casting spells within his/her capabilities tires and exhausts faster then one that casts beyond his/her abilities. And since hits are capped by spell Force it seems only natural to think that the usage of net hits falls within the mage's capabilities.
Anyhow, that's my take on it. YGMV