I'm sure your meaning .300, anyway each box really has it's own distinct std dev. Box 1 you'd have to agree with it having a tighter std dev say .240.
than say box 4 which has a naturally wider st dev. If you have enough data for a dog out of the same box say more than 10 starts you'd use that data and st dev (as long as no outliers occur). Sometimes you have to make synthetic changes to your data, to how you think the dog will perform.

Kennedys sheet is based on using a normal distribution, which doesn't happen each box for each distance will have it's own basic distribution, the only times that changes is say a wide runner starting in box 1 for example.

Only when dogs and for that matter horses race on their own do you get a normal distribution of times.
I remember conducting many trials of dogs on their own on a track, at that time was a replica of Wentworth park with the help of a great late trainer