The key here is "around" I think. Sure, it might be off a 1/16th, even as much as an 1/8", but that much won't affect your p'up plane. Do as Peter suggested and measure after the 16th fret and draw a line. Start there and go backwards until it no longer touches the body, which is usually right before the bridge/tailpiece. If your router still wants to dig in all the way to the bottom of the body, the strap button end, then you're putting too much angle in this plane. Mine usually work out around 1.2-1.4 degrees..ish.