NB time intervals (shutter speeds) have been rounded down for convenience.
The time interval progression, strictly speaking, is: 18.104.22.168.256.560 seconds.
In practice, amending 16 seconds to 15 facilitates conversion to minutes:
22.214.171.124.240.480.960.1920.3840 become 1/4.1/126.96.36.199.188.8.131.52 minutes
This ‘liberty’ is appropriate in extended pinhole exposure times due to the confounding variables of actual pinhole diameter, actual relative f-stop, actual ISO of the materials employed/reciprocity and accuracy of timing device used.