Decided to make some chips before going in to work this morning.
F360 lets you set up both-ways, conventional, or climb. For facing it defaults to both ways.
I got what I was after with conventional milling.
There was significant bow towards the middle of the piece, so it took three passes to clean up the canyons left by the first attempt. I ran the second and third passes in the y-direction after which I noticed some some very faint tool marks. Running my finger over the surface I could just feel a little ridge on the left side of every pass – so my spindle is rotated slightly out of square around the y-axis. Not enough to affect my next couple of projects, but I will fix it when I have time.
Thanks everyone for the help and advice.