Am I being too conservative in my analysis of rentals?

[Cosmo Lee]

Hi All,  

I've been around for a while now and I'm taking action steps in getting my first investment property. I created my own rental property calculator in a spreadsheet which is roughly based on the calculator on the site. When entering the expenses, I'm wondering if I'm being too conservative and thus it is skewing my numbers to show that a property won't be as great. For example, for insurance, maintenance & repairs reserve, utilities, and PM, i generally assign 10% of the monthly rent to these. I also do a 1 month vacancy reserve every year too. Property taxes I usually try to calculate myself, but I found that my calculation is too high most of the time than the figure that is given by the seller. When exploring the marketplace, I find my expense assumptions to be significantly higher than the seller's are posting when they post their ROI/CF figures. So I'm thinking a good estimate is probably between what they're giving and what I'm using. What percentages/figures do you use when analyzing a rental property?

Thanks in advance!