my understanding is that each late scratching is calculated individually and then added together
naturally the timing of your final field bet affects what outcome applies
TAB bookies for final field win bet scratchings calculate the deduction as follows
80 divided by the last price of the scratched horse
so if the horse was paying $10 the deduction is 8% ( 80 divided by 10 equals 8 )
firstly please allow me to apologise on the brodsters behalf for ignoring this portion of your question.
not sure this will apply to the brodsters betting (i imagine anything he touches would surely shorten too much to represent value a day or so later) but say you were betting to win 1000 then knowing the deduction % would allow you get some more on (subject to value) to retain the opportunity to win 1000.
that was on rising stars final night, he had finished a distant 4th in his heat which was by far the fastest heat. The first 3 in his heat were the first 3 home in the final. The other heat winner was tuapeka knight and he missed the final also.