I don't see any way to use variables for this. But I don't understand what you mean by this:
Then I want the blank field to reset to blank or 0 for the next time it's used.
Not sure what you mean by "It".
Except for that statement it sounds like what you want is this:
where the user enters a number in the value field and a running total summary field computes a running total for the values in that field.