The plate arrives and it's roughly the size of a hubcap. This is not a personal challenge, though restaurants sometimes make it feel like one.
Asking for a to-go box is the least dramatic thing you can do about it. It's also one of the few restaurant strategies that doesn't require a single calculation.
Do it early, not at the end
If you wait until you're full, the decision has already been made. Boxing half the plate before the first bite is just a way of choosing a portion while you can still think clearly.
Tomorrow's lunch is a nice bonus. It's not the point.
Nobody is grading the order
You don't have to pick the salad, and you don't have to skip the fries. Plenty of meals are worth ordering exactly as they come, especially if you're out with people you like.
A 10–20% deficit is a weekly-scale idea. One restaurant plate doesn't wreck a week, and one perfectly restrained order doesn't make one. The average over two or three weeks is the thing worth looking at.
Watch the extras that arrive free
Chips before the meal, bread for the table, a second drink, dessert because someone else ordered one. These don't feel like separate decisions, which is why they're easy to miss.
You don't need to refuse them. Just notice them, the same way you'd notice that the entree came with two sides.
Don't compensate on Monday
A big dinner out doesn't call for a tiny breakfast the next morning. That habit tends to produce a hungry afternoon and a bigger dinner, which is how one meal turns into three days of noise.
Go back to your normal pattern. If the weekly trend really has shifted after a few weeks, adjust once by 100–200 calories and watch again.
Keep the pleasant part
Eating out is mostly about the company and getting out of your own kitchen. A to-go box on the counter shouldn't be a reminder of anything except that lunch is already handled.