1. Justification not required.
2. LOI can say offer is based on information provided. In my 35 years, any LOI that says offer is based on EBITDA, FCF, etc. is rejected. Buyer can walk or renegotiate anyway. So, why upset the seller and why get into what is EBTDA, FCF, etc. I can find 50 different ways to define any economic metric.
3. There are many sources of market multiples, but you should run your own pro-forma.

David's described approach is how I'm approaching it in my LOI. It's something I've discussed with the seller so not a surprise to include it, and serves as a record from which to calibrate based on EBITDA adjustments, or any other issues that come up in diligence.