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As you already know, even when you're duplicating a published load exactly, i.e., each of the individual components is exactly as listed, you should start below the maximum and work up to be sure it's safe in your particular firearm. Should you then change any of the components, the safe and recommended course is to back off the powder charge by 10% or so and work up again. The same applies if you don't have the exact components to start with - just start with a reduced load and work up while looking for signs of excessive pressure.
I rarely duplicate all the individual components of a load recipe exactly, but have never had any problems following that procedure. In the case of target loads that are well below the published maximums, which is most of what I reload, I substitute components pretty freely without dropping the powder charge.
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