I thought about this afterwards (I got a partial) and reckoned if I had been expecting this I could have known just from time where I was on my track and used that as my fixed point and taken a minute or two to work things out properly. I could also have just curled back over the half way point (about 2 miles behind me) - I asked the Examiner later if this would have been OK and he said "Yes".
Not sure who you tested with, but, at PPL level this would be acceptable.
However, I'm not convinced a CPL examiner would pass this as it is not commercially expeditious.
A rough enroute heading whilst planning (obviosuly making sure clear of any danger areas etc,) and then firming up your plan and point of departure and plan etc.
(As a background I have given 1500 hours CPL instruction)