Low humidity is perfect when in flower. you want it under 50% for sure otherwise you'll be highly susceptible to Botrydis (Bud Rot).
You want to keep it low and try (if possible) to get it around 30% in the last few weeks.
The low humidity causes the plant to react and try to protect its self by producing more resin at the bud-sites!!! to protect the buds as it would assume it is protecting the seeds!!
Also take down the HPS at the start of the last week of flower and use a Metal Halide bulb (7-10 days) - This will increase the Psychoactivity of the resin and Fuck you up much more!!
Hope this helps dude!
...BudZ...