The cruise mode is activated when the airplane climbs to within 0.25 psi of the selected FLT ALT.
How sophisticated is the 737 pressurisation system? If the Captain has set a false cruise altitude, will the 737 even go into (pressurisation) cruise mode?
This statement from the manual seems to be saying that the aircraft is looking for actual pressure altitude being +/-0.25psi of target pressure altitude. If it never sees this, it may never go into cruise mode, but will still be in climb mode. This might be one of the reasons for not messing with the QRH.
I can perhaps understand what the Captain is thinking. If he sets the cruise altitude to below 28,000', then,
if the pressurisation system transitions to cruise mode, the controller will pick the lower differential pressure of 7.45psid (which, on the surface, appears to be a good thing).
Is the aircraft sophisticated enough to recognise level-off as an interim cruise mode? (like later generation aircraft)?
Anyway, we probably need more details to make better sense of this.
e.g. initial set target altitude, level off altitude, new cruise setting, , etc.
Regards
NSEU