Think about the peaks... US time ( im in Ireland ) 10am-1pm seems good here at DP for visibility of threads... so maybe same for Digg? Monitor the # of people on this forum over 2 days and you might get a very rough idea ..... Sorry couldnt help more!
This is not accurate. If you submit your story when people in US are falling asleep then chance is your story won't be seen by that many people. I like the idea of SparksFlying. Basically I just submit my story during the office hour on EST. I also monitor the amount of upcoming stories. If competition gets too tight I'll save it for later.