Agree 100% with vgpedlr!
I would add that a good way to decide what to eat is to ask, "Is there a healthier option available to me?" If yes, then choose to eat the healthier options. I think that is what we mean by "following the spirit of" the whole foods approach. A cup of broccoli and a sweet potato would be more filling, less processed and lower in calorie density. More nutrients, more fiber, less sodium.
If your diet mentality tells you, "But there's only X number of calories in the McDougall cup!", then I would say that that's one of those weird mindsets to let go of. Get that broccoli into your system! Get that sweet potato into your stomach! These are the whole natural foods that lead to improved bone health, improved kidney health, improved heart health, improved liver health (do I have to go on, and name every single organ system in the body?). You get the point!
When you are in a situation where the McDougall cup is your best, healthiest option, then eat it and feel good about yourself! If you are able to supplement that with a bag of (defrosted) frozen Brussels sprouts, then do that! If you are able to eat a few apples in addition, then I would see that as a way to improve the overall nutritional content of the meal.
But I would say, use that guideline of "choose the healthiest option that is possible for you in each situation". If advance planning opens up better options to you, then take advantage of those options. If life throws you curveballs and your options aren't perfect, don't skip a beat or lose your stride, just choose wisely from the available options.
This'll seem totally obvious to you in another month or so! But it takes a sec to get the mentality down.