Two alternative suggestions:(posted 8159 days ago)
1) Use T-max developer and develop normally. This developer gives an extended straight line portion to the film curve. The negs will be dense, but they should print normally, without the highlights blocking up; with just an increase in your printing time.
Option 2: Follow Ilford's recommendation for downrating HP5plus. Ilford recommend using Perceptol at stock strength. The times they give are for ratings of 50, 100, and 200 EI. The 50 and 100 EI times are both 9 mins @ 68 F; and the 200 rating time is 11 minutes. I guess you could interpolate the +1.5 stop time as 10 mins.
Good luck! Let us know the results.