Custom financial formula

[jb]

Hello,

I would like to add some financial formula like in fin.scm.
But if i add it into this file, i lose my modifications after a new  
installation of gnucash. And in fact,  i don't have the correct rights  
to do it.
How can i do in order to insure to not lose my modifications and in  
order to be use from several PC/Mac.

In generic way, it could be usefull if this type of file is located  
with my gnucash database file.
But what is the real solution ?

Best regards,
Jean-Baptiste.