Analyzing Properties, Budgeting Repairs

[Seth V.]

I am working on improving my methods for analyzing prospective rental properties to determine cash flow.  I am using the following numbers:

Vacancies - low in my area so I'm using 8%

Repairs  - 5% of rental income

Capital Expenditures - 5% of rental income + 1% for every decade over 20 yrs property age (i.e. 7% for a 40 year old house)

Management - 12%  (expecting 10% + 1 mo per new tenant)

So this adds up to at least 30% of my income.  Most of my other expenses are known and these are the ones that I am least confident predicting.  What do you think?  Are these good ballpark estimates to base my search from?