Long exposures need more time than the meter says. This uses each film's own datasheet: a formula when the manufacturer gives one, its table otherwise. Nothing is extrapolated past the published times.
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Metered 30 s
+1.67 stops
Open the aperture; the datasheet gives no corrected time for this.
Interpolated between two datasheet rows (highlighted below), on a log time scale. Not a published value.