About my maths, I did not do any, it was an estimation. Since you asked, I checked, and found this :
http://forum.shrapnelgames.com/showthread.php?t=17160
Which, coupled with the formula in the manual, tells me that with 17 MR and +8 penetration (and base 10), so you've got +1, so 62% chance. That's more than half.
With 21 MR and +8 penetration (and base 10), you've got -3 so 30% chance. Around 1/3. I ignore the elements you have not given me though, like skill in astral. Since I do not know about them.
The estimations I had were correct assuming the formula also is. That's where I'd be interested in the "real" formula for MR though, if the one of the manual is wrong - I can't find any reference to it on the forum. I must have missed it.
About counters... well. You will see. It's going to be WAAAAY cheaper than the one you found for the seeking arrows.