Oh so you went off measurements from another shirt. I think that is the best route. I looked at their size chart for a slim fit 'S' and
thank you for posting this, i will give it a try now!!
Btw, do you remember which collar you got? the 'Classic'? All these other options look bad/i am inexperienced with.
I would probably go either 'classic medium', 'button down med', or 'italian' MAYBE the 'cut away' collar