Jacob is the lead writer (alongside Francesco Nepitello, the lead designer of The One Ring) on the Rivendell pamphlet.
I've asked about a general "rule of thumb" for magical successes in the FAQ/Errata as it is clearly an issue lots of people struggle with. This has been a hot topic here, Facebook and on the discord channel!
According to the rulebook: "a Magical success is an outcome that can hardly be mistaken for an ordinary success — in other words, the Player-hero has achieved something so unusual that it could not be accomplished without a supernatural aid." Much as an ordinary success and a great/extraordinary success are different, so too is a Magical Success superior mechanically.
The "Extra success" icon is a good rule of thumb, but as Jacob notes it is important to also allow magical successes to do things a standard success couldn't.
Lots of the below comes down to reviewing what the Skill roll was trying to achieve narratively, and transforming a Magical Success in to a meaningful mechanical impact.
- If our PCs are climbing an old ruing wall as part of a Skill Endeavour, the Magical Success is at least another 6 on the attempt, maybe more. Perhaps I wasn't going to spend a hope of Hope on this, as I was saving them for later, but great to get a magical success naturally.
- Or if I'm trying to Awe some foes in to not taking part in a confrontation (whether a subsequent combat where they'd be extra foes or social rolls where large numbers or the "Numerous" Distinctive feature would impose -1d), the Magical Success lets me "Widen Influence" (p19) and affect more NPCs, making the subsequent scene easier. I might not have spent Hope on that, needing it for something else, but the Narrative from a Magical Success turns in to a tangible mechanical benefit.
- I'm in a combat, and I try an action that would normally take up my main action but roll a Gandalf symbol for a Magical Success, and can now "make haste" (p19) so the Loremaster lets it only count as a Secondary Action.
- You're making Travel rolls at the end of a Journey. Perhaps I wouldn't spend Hope to turn this in to an auto success, but treating it as an extra 6 lets me take off another point of fatigue or turn a companion's failure in to a success etc.
The above is largely just working off p19, without considering special things that only a Magical Success could achieve.