I learned about siddhi mantra and Atma Bija from manblunder.com and ravi sir. It's is very interesting methodology for fructification of mantra.
I like to know about tantric scriptural reference regarding that or Is it a new method derived from personal observation ?
Athma bija is very much an established procedure as per tantra sastras. Its not a new method derived. During poornabisheka the sadhaka is given a new name (diksha patta) and athma bija. Though, many old mandalis do not give these for everyone immediately after Poornabisheka.
The athma bija identifies oneself with one's own self.!! A very common example is "Hreem for Lalitha". Similarly athma bijas are derived. Sri Ravi sir has written a detailed article on this.