Once you open a record for editing, summary fields in FileMaker become undefined as the total will depend on the new value you enter in the current record.
You may be able to get what you want if you commit the record immediately after creating it (Easy to do if you have a button for new records). That will at least enable you to see the total before you enter a field to start editing it.
I'm wondering if you couldn't capture the current total in a global field during the execution of a script to create your new record. This global field (after a refresh window) would then display the summary total if you put it on your layout as a merge text object. A second script that fires whenever the record is comitted could then update this variable with the new data from your new record.