kintone developer network - Record Edit Event - Enable/Disable field edits
So, in order to manually enter a value in a calculated field, you need to create your own custom calculation in a field other than the Calculated field, such as the Text field.
To create a custom calculation in a field like the Text field, check the following page:
kintone developer network - Using custom calculations
Just make sure to consider how not to have the calculation to occur when you are manually entering a value.
I hope this helps.