What kills most Dutching ideas is the run of short priced favs getting up in spite of their questionable form.
So
Try & go for races where the Fav is say 3.50 or 4.00

Also field sizes can play a big part on the runs of outs, so try for fields of
10-11 & less works stronger than the larger fields , where more outsiders have a greater potential of getting up.

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Cheers.