Distance= longer is better 400 yards serves very well.
Ignore you windage (side to side) completely ignore it.
Vertical dispersion translates into velocity and barrel harmonic consistency.
Test only one component at a time or in other words start with x powder and y seating depth and do not vary it in the same test.
I add powder in 0.5 grain increments for large cartridges an 0.2-0.3 increments for smaller cartridges. Careful to keep them in the right order.
Shoot slow and deliberate all at the same point of aim carefully documenting each shot one at a time and when your done working up to max or pressure your shots will suffer from vertical stringing from velocity variations.
It's the spread of that vertical stringing that you can use to find nodes or groups of different charges that are the most consistent at producing comparable velocities.
They may go straight up or stagger based on either windage or harmonics and it is generally not reliable to assume which one.
They may climb then jump back down so it is important to follow along and not assume they are all climbing as the powder climbs.
You will see clusters of vertical and in order to stay away from the edge or border of those clusters you need to select your load from the core of those groups.
Use an oversized target to make sure you catch every round and do not attempt at 100 yards as the dispersion will not be pronounced enough to be definitive.