maybe you ought to consider using a film with better reciprocity charateristics than HP5. Tmax 100 is much better than HP5 and Acros is that much better than Tmax. Acros is linear out to 2min (really think about the implications of that&#33, where Tri-x begins to "depart" after 1/2 second.
Acros becomes faster than Tri-x at an indicated 2sec exposure. I often use Tri-x out to a 4 sec indicated exposure and switch to Acros for anything longer.