Sit up as straight as you can, with your back against a wall, and hold a book to the top of your head so that it rests on the very highest part of it. Make sure that the plane of the book is at a 90 degree angle with the wall. Then, without moving the book, mark the spot at the bottom of the book where the book makes contact with the wall lightly but noticably with pencil. Next, grab a tape measure and measure the distance from the floor to the pencil mark with a tape measure or meter stick. This should give you a pretty accurate idea of your sitting height.
The model in the picture you showed looks like he's done 10CM of LL with most of it in the tibiae (although he probably hasn't in reality), but he still obviously looks good enough that people are willing to pay him to model.
In my opinion, mock ups and having someone take pictures of you from a distance (with a neutral camera angle) while you wear shoe lifts that give you your desired height increase are the best ways to determine what you would look like after LL.