My Herron mm phono amp provides 42dB of gain, which works fine (to use understatement ;-) with my 5mV output London cartridges. I don’t know how Schitt came up with 30dB for Deccas, but remember, 5mV is the cartridge’s nominal output, not it’s maximum. Transient peaks may produce even higher output, right?
Another phono amp incorporating a setting specifically for Decca/Londons is the Andros by Zesto Audio, whose impedance switch includes 15K ohms (the cartridge likes to be loaded down, to electronically damp it’s high frequency resonance). Zesto designer/owner George Counnas told me he put that in the Andros specifically because of all his friends and industry colleagues who use Decca/Londons, a cult favorite!