How much should you typically aim to Cashflow per property? Why?

I think there are always other variables that come into play when discussing cash flow.  For example, I look for at least $100/door but on a 15 year note and in a C+ or better neighborhood.

Further, everyone calculates cash flow differently. I calculate cash flow as gross rent - 30%(vacancy, maint, and capex) - PITI = true long term cash flow.

As an example $1000 (rent) - $300 (vac, maint, and capex) - $600 (principal, interest, taxes, and insurance) = true long term cash flow.  And remember the principal and interest is based on a 15 year note.