The most forward button, on each side of the front, goes directly above the front crease. Makes the crease stand out a bit, and the trousers to hang nicely. There is some controversy, created entirely by me, over the next one. The official version says 3.5 inches aft of the more forward one. I don’t like them that way. I prefer only 3.0 inches. The greater the spread, the flatter the angle formed by the suspender tabs, and the greater the tension on the waistband. I don’t like that. If I wanted to crush my guts, I’d wear a belt.
The back buttons go 1.75 inches from the back seam (standard) or 1.5 inches (my way.)
A long time ago, the buttons were sewn on the outside of the waistband. That is, now, rarely done, on dress clothing. I don’t know why they were moved inside. We’re surely not fooling anyone.
Just some additional information. The buttons moved from outside to inside because of the loss of the waistcoat. Suspenders, also known as bracers, were buttoned on the outside because a waistcoat would cover them. When waistcoats went out of style, buttons were moved to the inside. The back buttons were moved because exposed buttons kept catching on chair backs. The front buttons followed suit later basically to clean up the look.