Toe-in can be confusing. Lots of people think their leg vise (or any face vise) should close with perfectly parallel jaws, like a veneer press. And in theory, that would indeed be ideal, but only if your work piece also has perfectly parallel faces. The reality is that its easier and more functional to make a vise with toe-in than with perfectly parallel jaws, with the bonus that toed-in jaws hold less perfect pieces as perfectly well as perfectly parallel pieces.
Here's why toe-in works. Think of your leg vise as a holdfast, with the moving jaw (chop) being the holdfast itself, and the leg your bench top. You would never place the pad of the holdfast right at the edge of a board, where the holding pressure is only on a very small portion of the board. Rather, you try and place your holdfast near the center of the board, to distribute the clamping pressure over a wide area. If your vise didn't have toe-in, chances are it would grab your workpiece, like the holdfast placed at the edge of the board, below the center of the piece, that is, closer to the bottom edge near the vise screw (where there's less holding power.) Then when you go to plane the edge of your board, it slips and pivots. That's because you're holding the piece at the farthest point (in the vise's weak spot) from where you're stressing the board. This arrangement puts the most stress on the board, making it very easy for the board to shift in the vise.
Toe-in moves the holdfast pad (moving jaw) from the edge to the middle of the board, and thus pins the board firmly to the bench top (vise leg.)
The other reason for toe-in is for workpiece stability. If you're working the board into the edge of the bench (dovetailing), and not along the bench (edge planing) you want the piece held at the very top edge of vise opening. If the piece were held lower down in the vise (lack of toe-in) the jaws will be slightly open at the top and your board can vibrate in the jaw during sawing.
Below is a customer pic of their Glide installation. The top of the bench will fall between the red arrows. The Crisscross is built with toe-in, and because of a number of factors, the toe-in will be slightly variable from bench to bench. But that's the beauty of toe-in. It doesn't need to be precise. As long as there is some toe-in, you're fine. The range is very forgiving. At the bottom of this chop, there's about a 1/2" gap, but once the jaw reaches the top 4" of the vise (the only area that holds your work) the toe becomes subtle. And that's a good thing, since once you tighten the vise down a bit, the jaw flexes slightly and spreads holding pressure over more of your workpiece, with the most pressure right at the top of the jaw--exactly where you want it.