Your script will modify whichever record is current the first time you access the layout where you set up this trigger. That is very likely to modify records that you do not want to modify in this fashion.
There isn't a trigger that is tripped only when a new record is created. Fortunately, entering a default value such as this does not require a trigger nor any scripting.
Open Manage | Database | fields and find the Overhead field.
Double click it to open field options.
Click the auto-enter tab and type in 0.12 into the data box.
Now every new record in the Overhead table will automatically receive this value when it is first created.