You'll have to edit the file "autoPrizes.xml" yourself, using Notepad or any other text editor. In it you'll see a section like this:
<levelSelection method="predefined">
<predefined countBuyins="true" countRebuys="false" countAddOns="false" />
<formula text="buyins + (totaladdons * 2)" />
</levelSelection>
Replace the formula text with your own:
<levelSelection method="predefined">
<predefined countBuyins="true" countRebuys="false" countAddOns="false" />
<formula text="buyins + (totalrebuys / 2)" />
</levelSelection>
Note that the TD app doesn't support re-entries at this time, so I've replaced it with rebuys.
To find the file, go to the Preferences tab and press the "Config Files" button. Select "Prizes Files" and then press the Browse button under "Automatic Prizes Configuration". That will open the folder to the autoPrizes file. You can right-click the file and select "Edit".