I have not use Neopan 400 so I can't give you any specific advice on it. However, I highly recommend you get a copy of 'The Edge of Darkness' by Barry Thornton and read it. It may open your thinking to about exposure and development. I bought a used copy off Ebay and the only section underlined by the previous owner dealt with exposure and changing the EI to suit conditions.
Thornton recommended shooting at box speed and developing normally for low contrast situations. For higher contrast situations, reduce the film speed by 2/3 stop and reduce develoment by 15%, for very high constrast situations, reduce the film speed by 1 stop and reduce development by 20%.
This is good advised to try out. If done it and don't have any complaints.