dispel chance

[WraithLord]

quote]Truper said:
If the dispeller's extra gems spent + skill in Astral magic (or perhaps skill above that required to cast dispel) + oeD6 is greater than the globals extra gems spent + oeD6, the global will be dispelled. So:

Very high chance of failure.

Less chance, but the odds are not good. (depending on how many less than 30 were spent)

Basically 50-50, with a slight edge for a powerful Astral caster.

Arbitrarily low, depending on how many extra gems were spent.

[/quote]

This raises an interesting probability question.
What is best cost effective strategy for dispell.
Two extremes, on the one hand trying to dispel with no extra gems. If attempt fails try again.
On the other hand cast dispel with a Hugh amount of extra gems say (relating to the example) 60 gems.
so gem wise three dispel attempts cost the same as one with 60 extra gems.

Another factor to take into account is that actually the amount of extra gems spent to cast to GE is usually unknown.
It could be anything from 0 to whatever.

I want to plot a graph of the chance of dispel dependent on the factor of extra gems.
A basis for this graph is probability of one oed6 to be greater then another independent one.
Say Xa = result of oed6, Xb = result of oed6, P(Xa>Xb)?
For regular d6 I think it something like 15/36.
For oed6 I am not sure. I think is something along the line of:
15/36 (sum of events of Xb less then Xa in first roll) +
1/36*(15/36 + 1/36*(15/36 + 1/36* ...
(event of first roll ties on 6 and then second roll)

I could be entirely wrong so if you are a math expert and reading this I'd appreciate any comments to my reasoning.