[mindlar]

From what I recall reading the manual the dispel chance is a random die plus extra gems plus five times the caster's skill over the minimum. The same formula is used for both the global and the dispel/new global spell.

From what I've gathered from the boards, casting at a site causes the extra gems between the bonus and the actual casting cost to be wasted.

For Gift of Health at a 20% off site:
40-50 gems are the equivalent of 0 extra
51+ are (gems-50)