Before the engines started but after the Auxiliary Power Units (which provide hydraulic power) were started, the engines were gimbaled to ensure that the thrust vector control system was working properly.
At T minus 4 minutes, the fuel system purge begins. It is followed at
T minus 3 minutes 25 seconds by the beginning of the engine gimbal
This difference was tracked by the Space Shuttle Program.
Each flight had a Propellant Inventory prepared for it. Here is an example.
The major prelaunch times tracked are
LOADED (blue arrow)
ENGINE START COMMAND (green arrow)
SRB IGNITION COMMAND (red arrow)
The mass at SRB IGNITION COMMAND is the mass that was important for ascent performance. (Note: ...