Display Percent rather than Count

Hi,

This seems like a simple thing to do in theory, but I can't seem to make it happen in Power BI.  If anybody has any suggestions, it would be greatly appreciated!  Here's the scenario:

• I have a Report that pulls data from an Excel file on Onedrive. 
• I have an ID field that I want to count - I use the Card visualization and change the ID field to 'Count' and I get the total.  Great!
• I now want to add a filter where Amount = 800.  I drag the Amount field to the Visual level filters, set it to 800 and I get a new total.  Also Great! 
• However, I need to report it as a percent of the Total.

i.e. Count of all IDs in the Excel file is 1000.  Count of all IDs where Amount=800 is 778.  How can i display it as 77.8% rather than just 778?

Soooo Confused.  Help!

Thanks!