Originally posted by scimmiaz 240 second exp? Of course, in the night?
Take the ND filter off and at f16 middle gray is EV9 (=1/2 sec at f16, 100ASA). That is not very unreasonable in the twilight after sunset, no? And after 120 seconds you're suppose to apply 1/2 stop of additional light for reciprocity (=f13 or 360 second exposure) with Acros but I didn't do that. Neither f13 or 360 seconds is what I wanted so I counted on the film's latitude + scanning and neglected reciprocity.
EDIT: What is interesting is the sum of exposure errors and the film's latitude and using an image editor. First, I metered for 100ASA with my Pentax Spotmeter V but really my EI is between 50 and 80. Second, a NDX400 filter should really be ≈8.6 stops if we believe the filter factor of 400 is accurate but I used 9-stops. And finally, ignoring reciprocity.