The 748 had the same 40 psi minimum for layshaft chatter, but I never did understand fully what layshaft chatter meant.
Milking the HP cock seemed to only be an alternative to using the fuel trimmers.
A positive engine driven force through the gearbox was necessary to prevent lay shaft chatter. A torque range between 40 psi and 0 psi could mean the reduction gears receiving prop forces instead of engine driven forces, presumably RR detected increase wear damage to the reduction gears/shaft.
Milking the HP cock was needed after trimming the fuel trimmers back to zero. A normal start, OAT less than 15c fuel trimmers set 100% prior to start. OAT above 15c fuel trimmers set to 50% prior to start. With either setting, as soon as light up occured the EGT would rise rapidly meaning in practise the first action was to trim back to zero, with EGT tending to continue to rise rapidly one had to ease back the fuel HP cock to prevent exceeding EGT start limit.
p.s. It is all a distant nightmare... I mean memory E.O.E. EGT may have been TGT.
p1fel