Income multiplier (compared to monthly rent)? 

When evaluating a property for purchase you incorporate the vacancy as an expense against the annual projected gross rent roll to offset erroneous income calculated by multiplying the monthly rents by 12.Hope this better answers your question.

So, you'd add 25% to their govt income, then use that figure to multiply by 3.

So, to look at the untaxed income in the same light and with the same calculations, you should "gross up" the untaxed income 25%.So, when looking at 3X the monthly rent, you first add 25% to their untaxed income, then multiply that by 3.An example: In CA, SSI income including the state supplement, is $890/month.  

However your longest time frame you have imagined to get it ready to move into, multiply that by 3.And you'll have driveways and sidewalks and landscaping to put in, which is enormously expensive, too.And in the meantime, you're paying rent somewhere else.  

Don't allow damage or graffiti or it will multiply.

I have yet to see a section 8 tenant that could qualify.There has been case law where tenants prevailed on the rent multiplier requirement by Sec 8 landlords.  

And even a high multiplier is no guarantee that the tenant can afford the place.

Although it is later aggregated and then multiplied by the business percentage, the IRS could easily multiply line 18 by the business percentage to calculate the amount that would be reported on Form 1099.  

@jamesTo expand on what @Linda Weygant said about the home office deduction no longer being an audit flag, the IRS now allows a "simplified" method where you simply multiply the square footage of the office by $5 and that's your deduction.One has to think that if the IRS is offering and encouraging a simplified method of reporting, it's not going to be the cause of an audit.