Yeah, dividing out a known portion size is a good hack that will probably help with accuracy. In our research most people's calories and error came from eating out where they didn't have these hints, but this is a good trick if you mostly cook for yourself!
Historically I would rely on the restaurant's printed nutrition info. But I don't really eat out often enough for this to matter.
I started eating half of whatever was served as an individual portion whenever I was at a restaurant and not home cooking. It's the thing that tipped the scales for me when having difficulty losing weight.