Questions on global spells

[Nappa]

Does anyone know the formula for dispelling global spells?

if it costs 50 gems to cast a global spell and someone spends 50 to dispell is that 50% chance of it dispelling? what if someone used 100 gems to try to dispell it?

When 5 globals are up is there a simple 20% chance of being booted when a new global is cast or does adding gems to the base cost of casting a global help you avoid being dispelled this way?

thanks for any help.