There's no exact way to predict since you'd need to know the sizes of each class between the one you were offered and the one you actually took, and the ages of every pilot in that class. Hypothetically what if you defer a class, and that class you skipped had every person older than you? Your ending seniority number wouldn't change. It would be 50 worse if a class of 50 were all younger than you.

Honestly your final number isn't that important in the grand scheme of a career when you're talking about skipping a class or few. What it will affect more immediately is how quickly you get more people behind you, as that will more directly impact your seniority progression off the bottom of the list. That will give you better schedules faster + some furlough protection if that ever becomes an issue.