6flat help with new value after rents raised please

[Nick Versetto]

I'm looking at a 6-flat that's listed for 600k with gross income of 68,400.  My question is if I raise the rents and bring the gross income to 86,400 what's the equation I use to calculate the new value?  

The reason I ask is because I'm going to attempt to see if the seller will hold a second and once rents are more in line I can cash out and pay him off.

Thanks,

Nick