Niagara custom scratch pad

hello
i am junior at Niagara particular and i have some problem with it
first of all i want to know is there any kid of tutorial about Niagara scratch pad ?
because most of tutorial i could find talk about very basic stuff that can be done with normal Niagara module
and now my problem i have a pebbels emitter than spawn like 2milion small rocks .i want to make a user parameter to control distance culling from camera .usually i change this in mesh renderer but this time i have tons of same Niagara system with only a little difference form each other .so i decided that instead of creating too many system for my each sequence , i would better of making one system with a lot of user parameters .so i figure out that i should make a custom scratch pad that do the culling for me and i assigned a parameter to that scratch pad .so i want to know how to make that scratch pad .
(sorry for my English iam not native English speaker)