If you measure from the floor to your naval and get 22.5 Then a 24" overall length kilt is going to be perfect for you.
A Traditional Kilt is worn at the natural waist which is just under the ribs. In the back the very top of the kilt should cover your short ribs.

I've heard a lot of stories of the kneel on the floor method of measuring. In actuality it is an old kiltmakers trick.
Here is how it works.
If you have your customer kneel on the floor and measure from the floor up to the natural waist that will equal the total length of the kilt.
But because the kilt will have the top buckles 2" below the top of the kilt it will lift the hem to just above the knee cap.
When you have the kilt on and you kneel the bottom of the kilt should be about 2" off the floor.

So forget the measure to the naval stuff. Everyones naval is in a slightly different place in relation to their natural waist due to the different amount of belly. If you have a large belly your naval will be much lower than someone with washboard stomach.
Use the natural waist. To find your natural waist take your hand and place it on you back just around your kidneys. spread your fingers so that they are touching your short ribs and just touching your spine. Then bend from side to side. Your natural waist will be the place near your spine that bends the most. Just under your ribs.
That is where the top strap is cinched into.