An 0.3 stop over-exposure is usually, in my experience, too small and inconsequential to be noticed with the latitude of C41 emulsions, including Portra that I have used. If the question was about transparency film, over- and under-exposure would be a more critical consideration. I would not be concerned about 0.3 or even 0.5, but run a series of tests at 1.5, 2 and 3 stops up and down. Many photographers set their exposure preference based on tests in given lighting conditions e.g. flat/overcast vs bright sun with shadows, and use their standing test results to achieve the outcome based on prevailing conditions.