Just to clarify, I am defining 'the crema fraction' as the emulsion remaining after 90 seconds, after the CO2 'fizzies' have bubbled away. I can have 100% foamie/fizzie stuff at the cessation of the shot, but the galactomannin-stabilized emulsion of oils that stays put is what I call crema.
If it took me 7-8 seconds from the time the pump started until the pour started, I would run the pump for 7 seconds, stop it for 5 seconds to allow expansion, then pull for 25 seconds. That infusion/expansion step reduces channeling and improves extraction in these lower end vibe pump machines.