How to calculate an offer on a multifamily building?

[Sergio Ramirez]

I have a landlord who wants to sell his building for $1M. I have seen similar multi-families in the area go for $875,000 - $1M. I watched a video on Youtube from Jerry Norton on this. In his video he states in order to calculate an offer you must focus on Net Operating Income instead of Comparables like you would for a regular house. I did an example deal from a different property where I was able to get the financials and rent roll to plug in the numbers in the formula provided by him in the video. I followed step by step and there was no guess work since the numbers where right there. Long story short the seller for that building wanted $10M and my MAO was roughly $225,000??? I know something is missing in the formula. The landlord who wants $1M for his property is motivated to sell but I just need to know how to calculate my offer. What information do I need and what formula is used to calculate this?