let x = atkDV * 16 + dfnDV
let y = spdDV * 16 + spcDV

Then for grass with encounter rate 25 either
floor(13 + 3/2 * y) <= x <= floor(26 + 3/2 * y)
or
floor(141 + 3/2 * y) <= x <= floor(154 + 3/2 * y)

It seems uniform within these bounds, but I'm not sure.

Graph: http://imgur.com/5Zqf5X2