Is this Sporran too formal for daily/casual wear?

[Randy McIntosh]

One of the rules of thumb about wearing a kilt that I always violate is that if you're wearing a sporran, don't wear a belt, or at least a wide belt. The top of the sporran should fall two inches or two fingers below the top of your waistband.

When and where did this rule of thumb come from? Not to wear a belt while wearing a sporran?

I would never ever wear a sporran 2" form the top of my waistband.