What you are getting into is reciprocity failure. This happens for all exposures beyond 1 second and faster than 1/1000 of a second. The spec sheet for the film, check the manufacturers web site on line, should have recommendations. Search this web site for answers as its been covered here before this post.

To compensate use a reciprocity chart and a stop watch. As always your mileage may vary and testing may be in order.

Steve Simmons recommends this for HP5 1=2, 2=4, 4=7, 8=19, 12=33, 16=50, 24=93, 32=145