Yes, it is possible to use fixed percentage for short breaks.
Go to Integrations > Price Optimisation > Beyond Pricing. Next to each property you will find a tick box in the Use percentages for short breaks pricing column.

When the box is ticked a Set percentages button appears.
Click this button.

In the following pop-up you can set the individual percentages for the number of nights.

If you have previously set up percentages for the short breaks for you properties, you can import these settings.
The percentages need to have been set up under Properties > Edit > Go > section Short Breaks before trying to import them.

Please also see this article.