I would have just said to the supervisor, "in my professional judgement,..." and been firm about it.
did the right thing. Just remember when it comes right down to it, it's your
nursing judement that comes into question when things go wrong and ultimately, it will be you
rationale for why you did or didn't do something if your practice or license ever came into question.
By definition, "A Standard of Care" holds a person of exceptional skill or knowledge to a duty of acting as would a reasonable and prudent person possessing the same or similar skills or knowledge under the same or similar circumstances.
So, as evidenced by the replies in this thread of reasonable and prudent nurses, you are not guilty.
Protect your patients and yourself first...always. That supervisor and your hospital will be no where around for you if your standard of care ever came into question.