I am probably not the best person to ask about spreadsheets. Yes they get the job done, but I am probably the only one to make sense of them, there is usually ********e everywhere....

But what you need is a column for:
a/ Dividend
b/ * for a Win
c/ 1/Dividend
d/ Win Count
e/ c(tally)
f/ d/e
g/ 100(base)/f

When you get 30 results into the sheet you need along with d and e, d31-d1 and e31-e1.
This will limit any changing of the base to 30 races.
You also need a basic understanding of if statements.

If you need further help I could probably whip you up 1 in a few days when I get home...
email is mohttnerb @ hotmail . com