So basically you could own high level spells but must fall within +/-10 for it to work or incur fatigue ?
Pretty much. That is very well illustrated by the table labelled "Formulaic Magic" on page 81 of the core book.
Not within +/- 10. There's no +/- element. The table on p81 is pretty clear. Just remember it doesn't apply to spontaneous spell-casting, and remember that penetration also matters if fighting something with Magic Resistance... and that creatures with Magic Resistance have their resistance affected by the auras they're in (including magi).
As an aside, when I was first looking at the base MR for relics or guardian angels, I was dismayed by how low the values were. But when you take into account the divine aura modifiers, those MR values start looking more and more impressive.