Probability of getting Marked

[Sprucery]

I have an XL 19 DEHu with one ring of +MR, so MR is 4*19+40=116. I'd like to know how probable getting Marked is.

It is always 125 (78 after stepdown). Characters with at least 178 MR are immune to this enchantment.

(50% success rate when MR=78)

Can anyone estimate what's the success rate for MR=116? Close to 25%?

(This would just be nice-to-know information. I don't think the answer would affect my actions much anyway...)

[Patashu]

What's the chance that 2d100 is greater than 138?

EDIT: ~19%

[Magipi]

What's the chance that 2d100 is greater than 138?

EDIT: ~19%

Where is this 138 coming from?

[Patashu]

What's the chance that 2d100 is greater than 138?

EDIT: ~19%

Where is this 138 coming from?

100(constant value)+116(MR)-78(post stepdown spell power)

EDIT: (((100+post_stepdown_spell_power-MR)^2)/2)/(100^2) = approximate chance a hex will work on you

what I don't know is, how to calculate post_stepdown_spell_power from monster HD...