How do you size yourself?

CoderGal

So I've been sized professionally and given a 34D
So I've been sized professionally during a short period of time when I weighed the same and was given a 30 B (boy was I muffin topping out of that one)
So I've been youtubing sizing and have been told:

Band size = length around your band when you are exhaled then you add...
3...
or 4...
or 5...

Which is it?

Cup size is largest part of your chest - (band size + random number above) = small number where A=1, B=2 etc.

I assumed it was this:
http://www.85b.org/bra_calc.php