Remember, the proto has blunted tips. The effective edge makes it ride longer than the actual length. So if the tips weren't blunted, it would probably measure to 158 or 158.5.
Since you would like a 158.5, get the 157 because it is basically that size.
I did not really mean it like that! I was JK on the 158.5. I'm just struglin' between a 157 which will be tighter to throw around or the 160 which will probably hold better at speed & jumps (relatively speaking that is).