new deal Risk Tolerance? using anticipated occupancy % to decide how big multiplex you should buy?

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im trying to decide how big of a multi family property debt i should be willing to take on.       ( MFH type of properties are something Ive been looking to get into)

so on a 4plex for example im assuming a worst case scenario occupancy rate of 50%. although I only look at good areas with realistic rents and I anticipate over 90% being the actual, I am trying to look at worst case scenario to ensure i can cover the monthly carrying costs if for some reason I had a couple units vacant at the same time or any unforeseen happens. 

Should I be assuming this, am i being to optimistic or to pessimistic? assuming that I need ANY of the rent in order to make the monthly bills of the property or should i only be looking at scenarios where i can cover all monthly costs regardless which would severely limit my buying ability? 

I know a 0% occupancy and even a 50% occupancy is extremely pessimistic, and again i am looking at properties that are already at or near 100% occupancy currently. Me being a newby to multi families I just want to make sure I can sleep at night regarding debt and not being overextended. I do have a decent sized commercial LOC which i could use if something major came up within the first month or two of buying, but again thats more debt, what should I assume?

thanks!!