so when you have your one meal a day how many calories are you looking to consume?
And how do you know how many calories you are consuming when you have that one meal?
It goes like this:
I'm aiming for an average of about 1750cals on weekdays and double that on my pig-out day (Sunday). Adds up to 14000cals a week or an avarage of 2000cals a day over the course of one week. Since my maintenance is about 2500cals, I lose 1lb a week. The 1750cals are 500cals for the 100g whey protein shake and 1250cals for the meal, so that's what I would be aiming for.
However, nowadays it all goes by experience, also because, as you mentioned, I just don't know the exact amount of calories I'm eating. If I notice that my rate of weight loss isn't correct, I just eat a little less or more the next week. It's not like it'll kill me if I lose 2lb one week and no pound the next.