That's right for a typical early 7 car train where there was a good proportion of business use, such as on the Eastern region. Subsequently on Eastern, that grew to 5 standard + RC = total 8, and then later 6 standard +RC = 9. On Cross-Country, it was normally 1 first, 1 buffet and 5 or 6 standard.
There were very early business formations that had both a restaurant and a buffet car, in separate locations not together, but that was only intended for services with a large breakfast or dinner clientele; they didn't last all that long. That reminds that there were early buffet cars that weren't intended as restaurant, but as time went on these I think were mostly converted to mixed meal service, that is, they could provide breakfast, dinner etc (but for less numbers than the 'real' restaurant cars) but also had a buffet counter. That's mostly how I remember the Western cars, certainly in the late BR era.
There were also variations in class for the half restaurant vehicles. Some were 1st, others 2nd. There was a general instruction that a half 1st should face the other firsts, and similarly with a half 2nd, that should face the other seconds. Sometimes on the routes with a smaller first demand, that meant 1½ vehicles would suffice, and allow more 2nds for a given number of cars.
