drainage area calculation?

we are going to drill a new vertical well wich is spaced about 600 m about the existing producer (producing at 9m3/hr with cummulative production of 260000 m3)* we have only DST data for this well (no pressure data after 3years of production)* the kh was about 160mdm (vertical homogeneous acting radial flow).we are dealing with tight reservoir* some drilled wells in this area has kh smaller than 30mdm and others greater than 100 mdm(nearest producer kh=160mdm)
I found my scaping too small*
I need hand calculation to confirm if I will interfere with the existing producer then I will drop my reservoir pressure below Pb quickly
please any precious help

I used a simple volumetric calculation using the following equation: : OOIP=Np=A*h*NTG*(1-Sw)/Bo=(pi*r^2)*h*NTG*(1-Sw)/Bo and I got the average current drainage radius (r)* I tryed also to plot 1/q vs Np/q* 1/the slope is the EUR of the well then i use the same volumetric equation to get the drainage radius.
Am I right ? Is there any other method to get the drainage area?

I am eagerly waiting for your replies.