- “一般来说，early/low 等于 20-23, mid 等于 24-26, high/upper 等于 27-29, 可是这不是固定的” (see here)
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"一般上" is more like Cantonese (that is spoken in the Guangdong province, Hong Kong, and also Malaysia etc). It seldom appears in Mandarin Chinese. If you want to learn the general Chinese, you can forget about that.
The usage in the second example is correct. As stated by other people, it means generally (but may not be always true).