Here is an example of the location for the boot insole millimeter length.
(It is typically located on the side heel of your ski boots)

To choose a mounting point here is a description of all the choices to help you choose what is right for your new skis:

Traditional Mount - This is the most common mounting point for all mountain skiers. This mounting position is most often referred to as midsole mounting position. If you are unsure where you would like your skis mounted, and you are a traditional all mountain skier, this is the most realistic mounting position for your bindings to obtain optimum performance of your new skis.

True Center Mount - This mounting position works best for skiers looking for ultimate terrain park performance. The midsole of the boot will be exactly the same distance from the tip and tail, so you will truly be riding the ski right smack in the middle. Center mounting a ski reduces swing weight for spins and provides the skier with a better platform for skiing and landing switch. Most symmetrical twin tips will perform at their best when mounted true center. Remember that symmetrical twin tips are geared for park riding, and backcountry freestyle skiing, so if you are desiring an all mountain ski, look for a non-symmetrical ski, and don’t mount it true center.

Core Center Mount - Core center mount refers to mounting the midsole point of the boot to the center of the skis contact points. This sounds confusing, but it really isn’t. Simply place the skis on a hard flat surface base down. Measure the distance between where the tip contacts the surface and where the tail contacts the surface. Then find the center point of that measured distance, and you have found your core center mounting position. This is a versatile mounting position for a skier who wants some park capabilities, but still need all mountain performance. Core center mounts are usually used on all mountain twin tip skis that are versatile enough to ride in the park all day, and still perform well all over the mountain. If you plan on spending a fair amount of time skiing park, this mounting position is a great choice.

+1, +2, +3, +4, +5 Mount - This refers to mounting a ski a centimeter of more in front of the midsole mark on the ski (or traditional mount). Moving the mounting positions forward from the midsole mark gives the ski a shorter feel when skiing tight trees or steeper terrain. Mounting the ski forward will also reduce the swing weight of the skis shovel and allow a skier to spin off jumps with less effort. Having the extra length in the tail will provide the skier with a better experience when skiing or landing switch, all while adding stability at high speeds when skiing the whole mountain. Remember, when you mount your ski in front of the midsole mark, the ski will have a different feel, and tend more towards a freestyle skiing experience.

-1,-2,-3 Mount - Mounting a ski behind the mid sole is going to give the ski longer feel, and is usually reserved for powder skiing performance. Unless you are absolutely sure that you are only going to ski powder on your skis, we would not recommend mounting your skis behind the midsole mark. If you feel like you desire a longer shovel for increased float in powder, simply buy a longer, wider ski with more rocker, and you will be surfing on top even on the deepest days.