What I was about to say. It really doesn't matter yet, but it's better to get the reader used to unitarity straight away. (Though I wouldn't explicitly mention unitarity this early -- I'd just replace the rule with "Multiply by 1/sqrt(2) when the photon goes straight, and multiply by i/sqrt(2) when the photon turns at a right angle" and everything that follows from that. If the maths gets too complicated with all those denominators, just make the initial amplitude -sqrt(2) rather than -1.)