Photograph the running race clock once. Everything else works out from that.
A race is (venue, T0) — both measured. Nothing to create, nothing to name: your photo of the clock says which race this is.
This never holds up the capture. Read the clock first and go watch the race — if the position arrives later, even from across the field, it attaches itself to the race then.
Any time during the race. The clock
already knows when the race started — we work backwards, so there is no
need to be there at the gun.
Fill the frame with the race clock, and keep any other clock out of
the shot. Reading a number correctly off the wrong display is the one
way this goes wrong.
Zoom until the clock fills the box. Reading it small is what goes wrong.
No confirmation needed — silence means it looks right. Only speak up if it disagrees with the real clock by more than a second.