http://www.wolframalpha.com/input/?i=c%3D1800;+x%3D-20;+c%2F(1+-+x+1%2F30)
If you use this link instead, you just need to put in CPI. Then in the shoot the beat! output parameters, you put the CPI result in there.
I also changed the scaling, as -30 seems to be a lot closer to double the distance rather than -20. I did a crude check. I screenshotted 0 and -30, and scaled -30 by 200%, and the center background portrait thing was pretty much the same size.