Many people over the years have calculated what kind of filter one should create for LSC feedforward use, but the most commonly referenced note on the calculation is an Initial Virgo note by Bas and others (Lisa links to VIR-050A-08 in LLO alog 13242, and I re-attach it here).  I have rewritten the calculation in Advanced LIGO terms in T1600504, which I also attach here. This note does not (yet) include the feature that we have of summing together two feedforward filters for the same degree of freedom - we use this for SRCL such that we can separately adjust the gain of the low frequency and high frequency parts of the transfer function, but it could alternatively be used to sum in the iterative "tweak" filter.