This page is for a more detailed description of all spells in the game, their scaling and widened effects.
Spells (copy from jobs, but list them in the "right" order...)
Magic x2,3,4,5 affects magic spells only. Like Fire, G Fire, Heal. It won't affect magic like skills like fire strike.
Widen on ability space, well, widens the attack. Like firestrike shoots 3 at once instead of 1. I think widen here doesn't stack. Also widen used this way won't have the below effect.
Widen on leading spaces doubles the effects of artefacts, this has both positive and negative inplications, it can boost the skill up artefact such that abilties are to max level 19 but make SP cost above 999 which makes the skill impossible to use. For the maximum ability damage (I had around 8630 fire), you need lots of artifact skill spaces with a 5x Magic, two of Skill up 4 and one casting time -40 and add a widen at a connecting junction. Widen here CAN be stacked, although not until very very far in the game will you be able to effectively manage it with high mana and maghic circles and stuff. Also widen used this way won't have the above effect.
Try to find the correct balance to achieve maximum skill damage.
Like start with 1 slot magic x5, then two skill up 4, and a casting time -40, and a widen at the end.
Then if SP cost is too high, 1 slot magic x5, 1 skill up 3, 1 skill up 4, and a casting time -40, and a widen at the end.
Then if SP cost is too high, 1 slot magic x5, 1 skill up 3, 1 skill up 3, and a casting time of -40, and a widen at the end.
...until you reach a SP cost you can handle.
Repeat with magic x4, magic x3, magic x2, until you acchieve maximum skill damage with the SP cost you can handle, then compare and use the best. Use a pen and paper to write down the maximum values so you don't forget them.