The range is based on changing the charge weight by a percentage, not by some fixed number of grains. That is because different chamberings have different case capacities, so the number of grains that reduce the pressure a certain amount in one cartridge will reduce it by a different amount in another.
The most common practice is to reduce any maximum load by 10% to get a starting load. Though it varies with the powder characteristics, that commonly results in somewhere in the vicinity of a 20% reduction in pressure (pressure changes exponentially with powder charge). Reducing pressure to ensure a safe level is what the starting load is for. But this means a powder with a 1.5 grain maximum in 25 Auto would have a theoretical starting load of 1.35 grains, or 0.15 grains reduction, which is a 10% reduction from 1.5 grains. That calls for more resolution than a lot of powder scales have. Also, note that Western Powders actually recommends reducing rifle loads by 10% from maximum to find a starting load, but recommends reducing handgun loads by 15% from maximum to find a starting load. I'm sure part of that is the problem with scale resolution, but it is a good practice with the exception of a few slow magnum powders which may not be safely reduced much without risking squibbing out and leaving bullets stuck in the barrel.
To reduce a load by 10%, multiply the maximum charge by 0.9 to get your 10% reduced load.
To reduce a load by 15%, multiply the maximum charge by 0.85 to get your 15% reduced load.