Before you can add own hours and amounts for one-day assignments for subsistence allowance, you need to activate the function on your company. Follow this article to activate: activate one-day assignment 

The settings for amount and deduction is based on standard amounts. If you want to add your own hours and amounts. Mark the box Custom hours and amounts and Custom food deduction. 

The settings you choose will now be the basis for the user's allowance and how the compensation is calculated. Press Save when you are done.