Quick questions on the 70% rule and estimating fixed costs

[Jeremy Hale]

I'm meeting with a wholesaler tomorrow to discuss some properties he has, one of which he has listed at 84k with a 179k ARV and 52k in estimated rehab costs. Of course, I will be scrutinizing these numbers and corroborating them myself, but that's initially what he has given me.

I made a quick spreadsheet using a combination of average values I've found online, along with J Scott's estimations, on the various fixed holding costs. Here is just a quick analysis that I did, the numbers on the left as if I financed it, the numbers on the right as if they weren't financed (feel free to tell me if you see any glaring issues):

http://imgur.com/GULWwYK

My first question is, am I correct in just eliminating the lender fees and mortgage payments from the fixed costs if I pay cash for the house? Is there anything else I should be changing? My money is in fairly low-yield investment accounts at the moment, so I'm not sure there's much of a point in adding in the lost interest I would be experiencing.

Secondly, why is there such a discrepancy in the 70% rule vs. my calculated costs? I realize it's a pretty hard and fast rule, but I wrongfully assumed it would be a bit closer to my calculated number. I thought I even estimated on the higher end for most numbers. Is the 70% rule just built around giving you a large buffer in case of unexpected problems?