I've seen two different interpretations of the rule: 1) Essence loss is rounded up to determine loss of Magic/Resonance. In this case, implanting anything, no matter how minor, causes loss of Magic. 2) Magic is lost for every full point of Essence lost due to augmentations, similar to how the difficulty in targeting a cybered-up character with magic increases. In this case, minor augmentations (up to 1 point of Essence worth) would be possible without adversely affecting the character's ability to use Magic or Resonance.

So which is it? Anyone have a link to a FAQ or errata I can reference, or know where I can find the rule in the books? So far I'm having no luck finding it.

From SR4a, page 177: "Anything that reduces a character’s Essence will also reduce Magic. For every point (or fraction thereof ) of Essence lost, the character’s Magic attribute and her Magic maximum rating are reduced by one. A character with a Magic of 4, for example, whose Essence is reduced to 5.8 has her Magic immediately reduced to 3 and her maximum to 5. Further Essence reductions do not reduce the character’s Magic again until Essence drops below 5."

From SR5, page 278, the example box at the bottom left: "Jimmy Two-Tone, who has a Magic Attribute of 4 and a maximum rating of 6, has decided he can’t live without some cybereyes. To keep the Essence loss low, he selects the Rating 1 model, which costs 0.2 Essence. His Essence is reduced to 5.8, his current Magic Attribute becomes 3, and his maximum Magic rating is now 5. He can get some more cyberware if he wants, as Essence reductions do not reduce his Magic rating again until his Essence drops below 5."

Excellent, tyvm, sir.

The fun part is when they start advancing it again.
It was a 4 but now its a 3 so when I raise it *again* to 4 it should only cost 20 karma
Unless it still counts as a rating 4 for sake of karma and you have to raise it to 5 at the cost of 25 karma to get your rating 4 back *I do this as it really makes my mage players think really hard about deciding to cyber up*

Except that's not RAW. The calculation is based off of current rating, not a "floating" rating. It's an interesting House Rule, but it is not RAW.

I know but it really cuts down some of the issues *namely the it's cheaper for cyber than it is for magic deal* which is why that part became it's own line.

Just keep in mind with that house rule it can also encourage more cybermaging by taking a bunch of cyber initially so their magic is just 1 or 2 and then start buying it back over time since the cost under your HR is just for raising from 1the low stat relying on their cyber to carry them along while they regain their mojo since even a 1 or 2 Magic can still give a lot of effects if the caster is smart.