Re: 20" barrel .308 max effective yardage
Here's a swag... perhaps someone has empirical data that's relevant?
You should be able to get about 2500 fps using Federal Gold Medal Match ammo with the 175g SMK from a 20 inch barrel.
That would put you at around 1180 fps at 1k. But, it will depend on your rifle and the actual loads you develop.
You can play with the numbers on JBM's website or using your preferred ballistics software. The 168g bullets will have a slightly higher muzzle velocity but will suffer more loss ending up pretty close to the same at 1k.