Hi Bazyle, I agree with Jason that dull red is a bit low.When I had a Parkray I used to get it burning fairly well and then get any cast iron into the middle of the fire, so that I could get as even as possible heat all around. I would keep an eye on it and watch for it to be a reasonably red to bright red but not cherry red, I would then leave it like that for a short while, depening on the size, and that pull it to the top of the fire and then dampen the Parkray down to the normal use level. Once the piece had gone back to black I would then pull it out of the fire and bung it in the ashpan untill morning. All the pieces that I did like this would machine very nicely.
Below are a couple of scans that I've dug out of my old college notes when I was doing my C&G advanced welding, which may help you.


Regards Nick.