For Results Reports: You can save default settings for your results reports by clicking on the settings wheel in the top right corner of your results page.
You'll see a screen like the one shown in the screenshot below. Edit the settings to whatever you want. Click on the arrow in the bottom left next to where it says "Apply my default settings," and then click on "Save as my default settings." Then the next time you want to update a report with your settings, you'll be able to click on "apply my default settings."

For your advanced Reports: Click on the setting wheel in the top left, or go to File --> Settings, as in the screenshot below.
You'll get a pop-up screen like the one shown in the screenshot below. Make whatever edits you want to the settings. Then click on the arrow next to "Apply my default settings" in the bottom left, and click "Save as my default settings."


For percentiles: I don't know of any way to calculate percentiles in the reports. I think you'll have to export your data to some other format (like Excel) and calculate percentiles there.