There's no exact science on power/energy/trajectory/speed difference. All the 15% talk mostly points to "minimum" differences... not average differences.

Recent message board studies by heavy conical users indicate that the variance is closer to 30% when using heavy, slip-fit conicals in larger diameter bore rifles. One thread at another ML website had several posts where 65 grains 777 were showing the same 100 yard trajectories as 90 grains Pyrodex P.

The best decider/evaluator is you at the range. Because your bore is different than your neighbor's exact same model (and) your bullets are bigger or smaller than mine (or) your powder may be the same but two years older & stored differently..... you see.. there are a host of reasons why you need to do range work for best results in your ML.