Help with vacancy and vacancy expenses calculation

[Robert Hesselmann]

I'm running numbers on some rentals. 

I'm assuming 1 month a year for vacancy rate, so If the rent is $800 per month I'm dividing by 12 and calling my vacancy expense $67.00 per month.

I realize that there are other expenses that pop up when a rental becomes vacant, such as lawn care, utilities, and some form of cleanup. I note that many people use 10% as a general average for vacancy. At the above rental rate, 10% is $960 or $80 per month, allowing an extra $160 to cover mowing, utilities, etc.

Is this correct?

Then the thought comes to mind that if I divide by 12, but have the rental vacant one month per 12, then I should be dividing by 11, no?

Of course the rental won't be vacant exactly one month per year, so perhaps over the 10-20 year hold time it averages out to only a few months of vacancy, and not 1 in 12.

Is that correct?

Thanks,

Rob