How far has the page been seen?
You can use the custom formula for analyzing this question.
Furthermore, the text parameter can be used as a dimension.
If the page is not scrollable and, thus, always shown completely, the value for the Scroll Depth is "100".
How far are pages scrolled, that have a certain page length?
Use the parameter "Page Length" for filtering this analysis.
Example: Take only pages into account, that have a minimum length of 500 pixels.
What percentage of all page calls have been seen for more than 50 percent?
For this analysis, we can use the custom formula.