I want to have a bit of a play around with a long night time exposure and I am trying to get a starting point.

The scene is to involve shooting towards a horizon that is what I would describe as light polluted (the majority of the lights are around 10 KM's away). The scene is also going to involve a person sitting (to create a silhouette, so I want to keep exposures within a reasonable sitting time (I get that there is going to be a bit of movement). With this in mind, I was thinking of shooting HP5+ (6x6) either at box speed or pushed up to ISO800.

I have had a look at the following website to get some information - and think that I am looking at around an EV value of -1 to 0 @ EI 100 (I tried to meter it tonight and my Minolta IV wouldn't take a reading). For how I described the scene, does this sound about right? If I am, I would expect a base exposure of around 2 minutes @ F11, converting the EV value up to EI 800.

How would I take reciprocity failure into account? According to the chart I have, @ box speed, HP5 + metered to 120 seconds should be exposed for 285 seconds. Would I use the same figures if I metered at EI 800 for 120 seconds? OR am I missing something?