Well the front legs are not even. Ultimately especially with a photographic view, it is ultimate to have the front legs so lined up that you cannot even tell there is another leg behind it.
The rear leg closest to the camera is over stacked. You want the hock to be Perpendicular to the ground as much as possible.
Sometimes it helps to put a leash and collar on the dog and put pressure on the leash backwards and throw a ball or toy in front of the dog (or have someone standing in front of dog grabbing attention to get ears forwards). This causes the dog to "lean" forwards causing the dog to lean more on its front legs more and shift weight which usually turns out to be the best stacks.
To give you an idea here is a standing stack and here is a leaning stack of Havoc
see the subtle differences in that topline? Leaning really raises the Withers and lowers the rear and makes the front assembly look much better put together.
also notice the slight camera angle difference. It is important to focus the camera directly parallell with the dog and aiming towards the center of the chest. Which you did a good job of, but I just wanted to point it out anyways since that photo is a good example.
Another thing to always keep in mind (not directed at the OP) is the sun. Have the sun shadowing behind the dog and not beaming down on a direct area. This can end up ruining a stack photo by washing out a dog, or glaring on a dogs coat which is never fun.
The last and most important thing is. Be ready to take a ton of photos. It usually takes me about 100-200 photos to get ONE decent stack shot LOL