Thanks for the suggestion. will definitely give it a try. I really liked guesstimation too. https://www.amazon.in/Guesstimation-Solving-World%60s-Proble...

There's this book I like called "Guesstimations in the back of a napkin" or something

This? https://www.amazon.com/Guesstimation-Solving-Worlds-Problems...

I read http://www.amazon.com/Guesstimation-Solving-Worlds-Problems-... and the attached link delves into far greater theory than the book.

Thank you for sharing this with us :-)