Questions about how Net Mineral Acres are calculated

New member and new beneficiary of mineral deeds. Trying to make sense of it all. I have a couple of basic questions.

Question #1: How are Net Mineral Acres calculated? (Example: 360 acres is 40 net acres according to a recent lease. How was that calculated?)

Question #2. I found a website OKCountyRecords.com. Is every mineral record on there for Oklahoma? I expected to find a couple of records that didn’t appear there even when doing a high-level search.

Thank you for any help that might be given. I am learning as I go.