How do I verify 50% rule?

[Benny Ng]

My goal is to purchase a multi-family and owner-occupy one unit. This will be my first home purchase. I've been using the 50% rule up until now to get an idea of what the CoCR would be, but I'm seeing some huge differences in property taxes zipcode to zipcode. E.g., one lists ~$6k for property taxes, while another lists ~$2k. That comes out to ~$300/mo difference in monthly operating expenses. How do I account for that?

I think the 50% rule is supposed to take property taxes already from some other posts I've seen (https://www.biggerpockets.com/forums/52/topics/163...). I figured I could just do a breakdown on per property on items such as property tax, property management (10%), vacancy rate (based on city averages). 

I'm getting stuck on how to estimate capital expenses though. One approach I was considering is taking the full cost for replacing the furnace, roof, etc. and calculate a monthly cost. 

Is there another way I could estimate this? Is this worth the time?