I would image that was the stylists idea.Lapel width should be calculated from the front chest width.For instance the front chest is 7”, half of that is 3 1/2” my usual lapel width. When you take off the seam at the armhole the lapel is wider than half the chest width, at that point.SooooIf you wanted to go for a narrow lapel I would say half the front chest width less 3/4”-1”
In the US in the early part of the 1960s some were less than 2 inches. In the 1970s they got clear out to the armhole, and some beyond the sleeves. Styles come and go. They last five years or more. Fashions are quicker. Younger people like fashions. My uncle and other guys in there 30s would get sent home to change from fsshion suit into a style (business) suit back in the 60s. The narrow lapels were style. The wide lapels were style, unless they went past the armhole. Shapes of trouser legs were all over the place too. One type of style is a plain jane. In the Old days tailors liked changing styles because it kept them busy. Tailors would tell men that they are getting out of style. "Those clothes are out of style." Looking at bums on the street and the you could tell the decade the trousers were made and the coat a different decade and the overcoat another decade- nothing matched. The plain Jane gets tiresome. This is why styles came and went. A bit of fun. A lot of older men heading towards the top brass positions where told, "No more fashions." When tailors made fashions they were far more interesting because of the individual details. Mass-production makes a million exactly the same. Sometimes marketers get involved and brainwash, "You like this." Some are good artist. Before mass production tailors made all the styles and fashions and other types of clothes. You can read about Henry Poole as a child bringing his school friends to his father's tailor house. These children were told to come in the back door, Saville Row. They were all wearing fashions. I read that he eventually had three different locations. Saville Row for the wealthy. Two other locations for the middleclass business men and the younger guys for fashions. 3 different doors to keep the different kinds of groups apart. Lots of books and lots of photos and paintings showing us that clothes are ever changing. For several hundred years tailors made them all.
I would image that was the stylists idea. Lapel width should be calculated from the front chest width. For instance the front chest is 7”, half of that is 3 1/2” my usual lapel width. When you take off the seam at the armhole the lapel is wider than half the chest width, at that point. Soooo If you wanted to go for a narrow lapel I would say half the front chest width less 3/4”-1”