Even with a modest pair of breasts it can be somewhat of a challenge to measure for a bra on ones own - so when the size and weight of a bust increase, so often does the challenge!
It often doesn't help that most online fitting guides tend to feature a model with a smaller bust, not to mention the fact that they are usually being measured by somebody else anyway (like You Bust It, You Buy It, for example, or Outgrowing the Oversize). Even plus-cup or plus-size guides rarely take the opportunity to use a model with proportions too far outside the realms of average, and so the procedure can be made to seem less of an ordeal than it actually is.
The biggest problem with even reasonably large breasts, when it comes to measuring at least, is that they really do seem to have a mind of their own. They slosh and sway - sometimes together and sometimes independently - misbehaving with every movement. Trying to keep a tape measure around the 'fullest part' of the bust, often in line with the nipples, can become frustrating to say the least. Even then, when you've got everything lined up, stood still and not even to daring to breath, simply looking down at the tape measure will somehow upset everything and it is back to square one!
Measuring the 'fullest part' of the bust also has some complications, especially as breast size increases. If our protagonist follows that advice, for example, she will also be factoring quite a bit of 'drop', as the size and weight of her breasts cause them to protrude not only forward and out, but also down!
There are a few more modern techniques for self measuring/fitting, but again these tend to have a drawback somewhere. Some retailers recommend to wear a soft cup bra that you already think fits well - but that only really works if you are expecting minimal change - the bra itself would probably be a better indication of change anyway. Alternatively, wearing a top that gives just enough support to keep the breasts higher on the chest for the purposes of measuring, but doesn't over-lift or constrict them, but once again, any substantial pair of breasts will more than likely be too much for an unstructured garment to hold. There is also the option to order a selection of bras in 'sister sizes' (that is, a bra size you estimate to fit you reasonably well, and then the closest sizes in both back and cup-size, both larger and smaller), and see which of those fits your bosoms and back the best - again with the flaw that you firstly need a good idea of what size you already are, and in this instance, need to be able to actually fit into highstreet lingerie sizes!
Judging from her sassy reactions, I don't think it will be long before our beautiful, full-bosomed protagonist gives up on both the condescending tutorial and the tape measure, plucks up the courage and goes to get fitted by a specialist...