Two major considerations in X-ray Computed Tomography are the required acquisition time and X-ray dose for the scans. In helical scan CT, scans are taken by moving source and detector in one continuous helix relative to the object rather than taking several separate circular scans. This not only reduces the acquisition time and dosage, but also allows faster processes to be captured and artefacts to be reduced at large cone angles. One theoretically exact reconstruction method for helical cone beam micro-CT is the Katsevich reconstruction algorithm. In this paper we look at two implementations of the derivatives required for the Katsevich reconstruction algorithm on flat detectors, both the original implementation suggested by Noo and a formulation proposed by Katsevich.