Supplemental rule: invoking legendary weightlifting acquaintance.
When talking to a more experienced lifter, always make reference to someone else you know that lifts much more than they do.
Use this formula: (amount they are lifting * 1.5) + 120lbs
Example of how to apply this formula:
How much you squatting, man? 400lbs? Sweet. Yeah, my cousin used to be into lifting and he used to do 720lbs. For, like, ten reps.