The Helpfulness ratings for your knowledge base articles can be found within the Knowledge Base section. View all your articles at once or use the searching/folder options to filter to specific ones. In the grid, you'll see a column for Helpfulness. You can click on this column header to change the sort order from most to least helpful or vice versa.
When viewing these numbers in the Portal, your users will see a percentage of helpfulness, in order to protect user-base privacy (how many people are reading your articles.) However, when you review the helpfulness in the Management Interface, you'll see both a percentage and number (eg. 4/4).
I also like your suggestion about a "No" answer triggering a feedback box! I'll pass that along.
Hope this helps!