dispel chance

[WraithLord]

I read some older posts regarding dispel.

I wasn't sure about something so I'm bringing it up again.

I will use an example as reference.
Say someone cast a GE that costs 70 gems and added 30 more for strength.
So...

what is the chance of failure if a dispel is cast with no extra gems?- is it 100%?

what is the chance of failure if a dispel is cast with less then 30 gems?- is it 100%?

what is the chance of failure if a dispel is cast with exactly 30 extra gems?

what is the chance of failure if a dispel is cast with more then 30 extra gems?- Is it a guaranteed success (0% failure) or a function of the added gems?

TIA all experts.