Usually, the vertical stroke that runs through the center of a word takes precedence (such as the middle stroke), followed by the vertical stroke (such as the third stroke in spring) as the main stroke. But it is not absolute. For example, Sichuan is the third one.
If you like calligraphy, you don't need to care who is the lead writer. After a long time, you will write a certain word seriously, intentionally or unintentionally, and it is the main pen.
The only thing to remember is that a word can only have one main pen. Others are main pens or auxiliary pens.