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The correct time and temperature are important. But you have to modify the listed development time to your own situation. You may be too heavy or too light on the agitation and there could be some other factors. Some people prefer a thinner negative, some like them more robust. I attended the Leica travelling school years ago and they said the ideal negative was one that you needed to print with a #4 grade paper. That's way too thin for me. I think they meant "in theory" the thinner neg is best. Kodak used to fib about the speed of their b&w films so the majority of people would overexpose. The idea was b&w take better to overexposure than underexposure and by overexposing, at least the customer got something. The guidelines are only a starting point. This is my input from spending 50 years in darkrooms.