Doesn't really work that way. No real point/ml increments. There is a spreadsheet out there on the net somewhere (I could find and post mine if needed), showing ml's to look for @ a certain point spread. For example, with a 3 point line, you want to jump on favs less than -145/ and dogs more than +120. You're not going to find this all the time, of course, but thats where you want your money.
But to answer your original question, the best correlation between the 2 is -140 every three points, but believe me, thats not exact..