Before you determine the mounting height of your wall guard you need to decide whether you will be using one or two wall guards.
Mounting Height for One Wall Guard
If there is one wall guard mounted near the floor it will normally be around 12" off the finished floor (measured to the center of the wall guard/crash rail). This can vary a few inches depending on the height of the baseboard and width of the wall guard that you are using.
Mounting Height for Two Wall Guards
If you are using two wall guards/crash rails, the first one will be around 12" off the finished floor and the second will be around 36". Again, this will depend on the width of the wall guard as well as what types of things you are looking to protect your wall from.