Clearly repechage's eyeballs work better than my software then... I realized the bike is at a slight angle to the wall, which would have skewed the angle difference down. At any rate, measuring is measuring. It's not like a tiny amount of top tube slope will make it explode or anything.
It is kind of interesting.
I'd probably use a bubble level across the bottom of the top tube to create a horizontal line, and then measure up with a tape measure or better yet stick.