Well, first of all let's assume that you are wearing your Kilt at Traditional waist height.

To find your natural waist place your hand with the fingertips just at your spine and about the height of your kidneys. Your thumb should be along your side just under the ribs . Now bend into your hand and you will feel an indentation where your thumb goes in just under your ribs. That is your natural waist and that is where the top straps of your kilt cinch into. It is also the height of the Kilt Belt.

Now strap on your Waistbelt at that level. Then put on your Waistcoat. The front notches of the Waistcoat should come to just the bottom of the Belt buckle.
(If you don't have a traditional Waistbelt and Waistplate use any belt and measure down about 1.5" below the center of the belt.)

Now to the position of the sporran. Remember, the sporran is not a crotch protector. It should rest lightly on the stomach. If it is worn too low the strap will catch under your belly and cause a pucker in the aprons. When worn properly the top of the sporran should be about at the level of the naval. This allows the sporran to hang free from the belly and not ride under it.