I need help reading my new salifert test kit!!!

[glb]

I just got this new test kit and I want to make sure I'm reading it right. For the higher levels, do you put the vial directly on the white part of the color chart to read it? Doing that vs. holding it over the chart makes a huge difference in the color. I know you're supposed to hold it against the card for lower ranges but the directions for higher ranges aren't as clear. Help!!!

 

[glb]

It's for nitrates. Silly me.

 

[glb]

Ok I found a video. It wasn't in English but I figured it out. My nitrates were 25 before a 2g w/c and about 20 after. It's a 12g tank. Oh no!! I cleaned all the sponges and filter media. I could be over feeding. I'll cut back. It may have been this high all along and I didn't know it because I was using API. How much and how often should I do water changes to get it down???