Another way of keeping the developer warm is to use a tray with water, of the same size - then the warming water does not get cold as fast. I have tried OB but I have never seen any big differences, so I just skip that for the most of the time. As Dan above, I start out with a set time (in my case 90 seconds at f8) and then I adjust as needed by eye. The most useful thing I have is the red led torch I bought from RH Designs. Maybe expensive, but it has been super handy for the Fomabrom papers.