tbh i juse use the numbers closest to my fingers. I'm lazy and don't care for rounded numbers. Generally its like.. 34 for opacity, 43, sometimes 23 or 24 if I am feeling adventurous. People who go past 5 are witches.
4, 8, 15, 16, 23, 42 i tend to use 100, and 0 a lot (flow). brush is always at 100, unless i vary pressure w/ opacity. no magic numbers for the layers, just whatever works. Just at the least i have 6 layers with the above numbers in each psd.