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.
Calculate
Metered 30 s
1 min 26 s
Same as opening up 1.67 stops (by time)
From the datasheet formula Ta = Tm^1.31.
Ilford HP5 Plus: datasheet values
No correction from 1/10000 s to 1/2 s.
Beyond that: Ta = Tm^1.31 (Ta = time to expose, Tm = metered time, in seconds).
Source: HARMAN technology HP5 PLUS Technical Information (2018-11)