Sorry, but it does depend on the question type.
You wrote "So a 3 mark question (with 33% penalties) has -1, -2 applied at try 2
and 3 respectively. The problem is when you have partial marks coded. So
I have one example where a student, on her 3rd try, got an answer that
would have scored 2/3 if she gave it on her 1st try. The penalty though
meant she scored 0, even though she actually worked out a 'valid'
answer. My original thought was that the adaptive mode would give 2/3 -
66%, which is 0.68/3."
Please export that question and attach it to your next post so I can try it out.