There isn't really a widely-accepted standard for severity, other than the academic impact, or intensity of intervention. I would think of levels of severity conceptually more the way that the AAIDD and DSM-5 define intellectual disability, which is not only by a certain cognitive profile, but also by the level of support needed.
The intensity of intervention tends to be more aggressive the younger the child is, as, after you've thrown the kitchen sink at them for many years, whatever limitations remain are usually the ones that are quite resistant to intervention. (Obviously.) At some point, one has to start looking toward the long-term, and providing access to life activities that have value to this individual, rather than remediating specific deficit areas to normative levels. That point may come very early (as with teaching keyboarding to dysgraphic elementary-age students), or much later (text-to-speech becomes appropriate for some students only in late high school/post-secondary, when the vocabulary of instruction takes a steep climb).
I would not worry about the exact level of severity. Instead, I would monitor progress. If whatever interventions are in place are helping her rate of progress to accelerate, then you are on the right track. If she is not beginning to close the gap/obtain functional skills/gain full access to areas of strength, and continues to experience disproportionate frustration or negative self-concepts about learning, then it's time to modify or re-think the supports and interventions. Identify a metric that can be more-or-less objectively and reliably collected with some level of frequency (like every two or three months, or more frequently), and start charting it. If there is a good IEP in place, the progress reports should be tracking a measurable objective of this kind. If you don't have measurable, achievable objectives written into your IEP goals, politely but firmly inquire until you do. Those are how you hold the school accountable for servicing your child's needs. Often, you can just keep sending it back for revisions until you are satisfied, and then sign it, rather than rejecting it on the first draft. Or you can accept specified parts of it, and keep revising the rest of it until you reach consensus.
