Retirement planning

Thoroughly enjoyed your webinar this evening sponsored by the local Chicagoland libraries. My question has to do with a general rule of thumb that I’ve seen and heard in various publications and seminars regarding the size of your portfolio( nest egg) to sustain a comfortable retirement for approx 30 years. For example, if your yearly expenses are 120,000 and social security for both spouses covers 70,000, the size of your nest egg should be approx. 50,000×25=1,250,000. In your opinion how accurate is that assumption as a general overall guideline.Look forward to your reply.
Thank you,