I am a homebrewer and have been concoting a recipe for a Chocolate
cherry Porter.

Last week I saw an episode of "Good Eats" on the Food Network. The
host, Alton Brown, was featuring recipes made with Cocoa powder. I
have attained the recipe from that show for Chocolate syrup. I plan to
add a few ounces of this to my beer. I was wondering if I was to add
some cherry extract to this syrup how much should I add for a nice and
subtle cherry flavor.

The recipe for the chocolate syrup is as follows:

1 1/2 cups water
3 cups sugar
1 1/2 cups Dutch-processed cocoa
1 tablespoon vanilla extract
1/4 teaspoon kosher salt
2 tablespoons light corn syrup

This is all simmered down to a nice smooth consistency. I know I am
probably asking a very simple question but I want to be sure I get this
right. Also, any recommendation on what brand of cherry extract?

Thanks in advance

Ryan