DSCR Ratio - how is NOI estimated

[Christopher "Bear" DeSanctis]

Considering that 

DSCR = NOI / Debt Service

and 

NOI = Gross income less operating expenses

When institutions are looking at operating expenses for a new purchase, how are they calculated?  

For example, if we know the gross rents, how does the institution estimate operating expenses, vacancy, etc for the purposes of calculating the NOI so that they can determine DSCR? I suppose one method would be to use seller reported figures but that wouldn't be prudent for many obvious reasons. So how do the banks estimate OpEx? Is there a standard multiplier like say 20% of rent goes to vacancy and opex? Or are there other ways it is done?

I am looking at structuring a deal and I'm worried the DSCR will be too low and the deal may benefit from restructuring by purchasing the 2 vacant out of the 12 otherwise occupied units with hard money as opposed to trying to purchase all 12 with a institutional loan just because I'm worried the 2 vacancies will drag down the DSCR for the new purchase.