Thanks Sandy. It is helpful. It won't take long to home in on the right time. My choice of .5 was just guesswork, since i don't do such measurements. My PMK times are shorter than normal for some unknown reason (not the water quality). I'm hoping that Pyrocat-HD will be more consistent for me from batch to batch. I currently have three lots of PMK all different, all carefully made at different times using chemicals from the same supplier.