I had mentioned here once before--A millwright plumber I worked with years ago once told me "The two thing you have to know to be a plumber are that water runs downhill, and don't lick your fingers."
I ran into this (below) on some DIY website (prohibited website posting)-it is not captioned, nor discussed in the text that I can find, so I don't know why it is shown--the implication however is that's it is an example of how to do it...
By the picture it looks like the drain in the wall is pretty high. Their both trapped. If you put an end waste on I
think you might be to low. As far as I'm concerned this is done correctly.