But if you have a super large main meal then the 2 small meals can still be large as they are still less than the main meal.
I just use common sense. I know some people who apply the letter of the law and expand their main meal to twice what they would normally eat. I go by the yardstick of a main meal I would "normally" eat, though some people have a lot of small meals and rarely eat one super large meal, so perhaps it's more like something that would constitute a decent and satisfying main meal for an average person.
It's not an exact science using gram scales, calories counters, etc. To me that borders on a scrupulosity that's reminiscent of the Pharisaical attitude. I try to keep the main meal within the limits of what I consider a normal main meal that a normal person might eat, and then just visualize the two small meals to make sure that together they don't rise to that level of a completely satisfying main meal.
Point is to perform mortification not to be all scrupulous about it.
If you double the size of your main meal so that your two small meals could be normal meals, that fails the test of mortification.
Nor would it be mortal sin of any kind if on one or another day you miscalculated.
Also, the main meal isn't a relative thing. I've occasionally done something where, because I was in a hurry, where I ate a pretty small main meal on any given day. That does not require adjusting "down" the other two meals on that particular day. Main Meal should be based on a hypothetical main meal that a normal person might find satisfying and not leave them hungry, not the actual main meal you ate on that day.
Mortal sin would be, "Forget this. I'm not fasting." If you trip up a little by an unintentional miscalculation, it's maybe a venial sin at best ... if you were negligent in keeping track. I'm sure I've had days where I had a third small meal where I had forgotten that I had already had 2.