Varying charge weights will mess with the Extreme Spread (other factors will do this as well). An Optimum Charge Weight load is more forgiving for small variances in case volume, neck tension, and inconsistent COL etc.
This requires the correct powder for the components used.
A 308 Win as well as others (considered an inherently accurate cartridge) exhibits this.
OCW load will give the best chance of small groups. However, if the load is not on a vibration node groups will not be as small as they could be, excluding other mechanical factors. So you need an OCW/Optimum Barrel Time load.
Back to the measure, I use a Lee dipper dumped into a pan on the FX 120i digital scale and trickle up, variance is 0.00gn/0.02gn.
This is a slow way to do it, but gives very good results for range work and competition. I also use this method for hunting rounds.
At the minimum many 1/2 moa groups (at least 5 shots) at 100 yards will more or less prove that your rifle/scope mechanics are good. At distance your scope must hold zero and track reliably.
To prove a good load for hunting or competition push the distance out to 300+ yards, once you get good 5 shot groups move to 10 shot groups. This will show you a load that will stand up at distance.
The Extreme line of powders are quite good but it is the air temperature that messes with POA/POI from day to day.
