Critique My Offer Price Model

[Joe Fornasiero]

I'm looking for input/suggestions on the model I use to come up with an offer price

Sample:

List price $135,000

Monthly rent               $1,350

Desired cashflow:     -$200 (Concept derived from the book Profit First)
Vacancy @8%:         -$108
Maint. @10%:           -$135
Capex @10%: -$135
Prop Mgt@10%:      -$135
Insurance:                -$37
Taxes:                      -$190
--------------------------------------------------------
Total for P&I:            $ 410

Use website http://1728.org/calcloan.htm and solve for Amount

Inputs:
Years: 30
Rate: 4.25%
Payment: $410
----------------------------------------
Offer Amount: $83,343.52 (subtract any rehab/closing costs from this number if necessary)

The one item this model doesn't take into account is the neighborhood. The individual should be able to determine the desired cashflow after factoring the area, type/class of neighborhood, and potential opportunity for appreciation.

Is there anything I should change in order to increase my success?