You'll lose a mark or two for each mistake you make (depending on how important the calculation is).
However, you won't lose marks in the audit trail if you describe what your model is doing accurately. That is, if your audit trail description matches what your model is doing, you'll get marks in the audit trail, but will lose a mark for the incorrect method.
The worst thing you can do is have your model wrong, and describe in the audit trail what it should have done if it was right. This is much more common than you might think, and most often happens when students copy & paste formulae from the question paper or guidance into their audit trail.
Obviously, the more communication you have in your audit trail, the better - so if your model is wrong (and you know it), in your audit, describe what the model is doing (the wrong way), and mention that you know/suspect this is wrong, and should be checked/corrected.
There are general overall marks for things like "is it easy to pick up errors in the model from the audit trail" and "is each step clearly documented" where you're more likely to catch up on any errors in the model itself.
As a previous poster said - the key to passing the exam is communicating what you've done.
