To amplify what Texas Range Ammo said,
here is Accurate's reason for always starting with the starting load from their FAQ. Interestingly, they give another reason why, unintentionally, with
another FAQ entry, and that is change of powder data over time.
My own main reason comes from the SAAMI standard. In it they show that the copper crusher, which is still used for much of our powder data, really can't be trusted not to be in error by as much as 25% or so. A 10% reduction in powder charge brings about a 20%-30% reduction in peak pressure, depending on the powder and cartridge and other component choices, so that averages about right to allow for the copper crusher error. The Piezo transducer cuts the error in half, so maybe one day we will be able to use a 5% reduction to get to a start pressure, but for right now we aren't there yet.
