If you are having trouble finding the center of your apron you can try taking the kilt off fastening the straps as normal, and folding the kilt in half with the top straps opposite each other and even. Then look at the outer apron. Where it folds is the center.
The convention among kiltmakers is to have a prominent stripe from the Tartan centered in the apron. So, when you have the kilt on you should be able to look down and check that the prominent stripe is centered on your body.
Your sporran should also be centered. So there should be the center prominent stripe running behind the center of the sporran.

Only if the center of your apron does not have a prominent stripe would you put the sporran between the stripes.

This gives you a quick way to glance down and check that everything is centered and aligned. Stripe of kilt centered, Belt buckle centered on the Stripe, Sporran centered on the stripe. No more "Honey, is my kilt straight?"