I photographed this at the very end of blue hour late in the summer. The scene metered at F/ 11.5 @ 2 min. Using a phone based app for Android OS called Reciprocity to calculate reciprocity failure. An exposure time of 3 min 45 seconds was suggested. My actual exposure was rounded to F/11 @ 4 min.
I’ve observed that reciprocity failure is less of an issue with Ektar below the 15 second mark. If you rate the film at 1/2 box speed or open your exposure by a full stop in the scene you should have a good negative to work from. Beyond 15 seconds there is still more research to be done.