I apologize for not tracking down the official SILAC citation, but within the recent 2014 Cox et al. paper on LFQ, there is a brief mention that a similar normalization approach is taken with SILAC data (both are precursor quant so this isn't too much of a stretch). I assume a better reference is within.

EDIT: I would also post this question on the MQ google group, since Jürgen (or someone else from MQ) will answer questions. This week is MQ summer school, so maybe not a super quick response.