I am actually not very good at just running numbers and coming up with an offer. I have to walk through a property, pull comps, then look at the numbers. Just looking at your numbers lets say that you can get a 30yr fixed at 7.5% on a commerical loan, your PITI will be $6518.33 per month. Not sure where you got $49,500/ $4125 per month.
Actual Debit service = $78,216
Rents $89,100
OP expenses lets say $30,000 per year and this is on the low side.
Leaves you with $59K or $4961 per month. Current price leaves you at neg. $1557 per month.
So if you expect to make it successful you would need to get the place for $395,000. This would leave you with a PITI of $3721 and a cash flow of $1261 or $114.00 per unit per month.
Yes, that seems pretty nuts to walk in and offer 395K for a property that someone is asking 795K for, but do you see any where I am hosing up the numbers? Remember, you don't count the DP. You have to base it off what you are paying for the property.
You could do the 70% rule based on comps and see where that gets you, but only use that if I am going to flip the place.