Using Simple Splat Mode, you can quickly setup new rules or modifying the existing, using 3 parameters
Min/Max Height is the normalize range of heights that will be influenced by the rule. Normalization means that the top value 1 is always relative to the highest point in your landscape.
For example if out range i 0.8 – 1 when you apply that to different terrain, it will be always between 80% and 100% of the height range
Min/Max Slope is the normalize range measured in angle that will be influenced by the rule. Range is between 0 ( flat area ) and 90 ( top steepness )
Weight is the contribution of that rule in case there will be more rules on same areas. That will allow a deep control despite the simple setting
You can enable the height gizmos to have a better view