Using that free Market Re-Framer
One can set the New Market Value box, at the top, to the required percentage & go from there.

You will notice that most newspaper & online pre-post markets are set around 125-136% before scratchings .
This is done to allow for possible scratchings of the longer priced runners, which is common to see.
Not so good if the first 2 Favs are scratched .

That Market Re-Framer is a handy bit of kit, for this exercise .
Its very well laid out & user friendly.
There are some other handy free punting tools there as well.

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Cheers.