For the first question, place Segment and ID on the rows shelf. Then, create a calculated field similar to the following:
total(sum([Number of Records]))/total(countd(ID))
**Note: to use countd, an extract needs to be taken
Then, place this calculation on the view, right click and select Compute Using-->Pane (Down).
For the second question I'm not sure exactly the desired outcome, but editing how the calculation is computed may help resolve it.