I can't say I could quantify my initial ER buffer using the common terms described by others, for several reasons. I was moving quickly toward being able to retire at age 45, back in 2008.
One reason was that I was best vaguely familiar with terms such as SWR and multiples of expenses. Given that I was planning to ER at 45, I would only have unfettered access to the taxable portion of my overall portfolio for the next ~15 years, at which time I could access my rollover IRA (after leaving work and converting the 401k to it). So, a SWR using my entire portfolio would not really mean much. Similarly, a SWR using only the taxable part of my portfolio would not really mean much because it would count for only 15 years.
I did, however, have access to two important things. One was Fidelity's RIP program which, with the help of my Account Executive, I completed and ran it. In early 2008, my AE gave me a green light to ER, the first outside evaluation of my ER plan. Two was my own spreadsheet which looked in more detail at the next 15 years of projected income and expenses, after I did the rollover IRA and cashed out about $300k in company stock.
My plan was to have the projected income from the $300k now in a "big" bond fund cover or just about cover my projected expenses. This would leave the remainder of the income generated by my taxable portfolio act as a buffer. This represented a buffer of about 25%. My spreadsheet had this buffer shrinking over the 15-year period to the point where it might disappear and I'd have to tap into principal the last few years. That was fine because once I turned 59.5, the first of my "reinforcements" would arrive. Those reinforcements are the aforementioned rollover IRA, then SS and my frozen company pension a few years later. IOW, just get me to age 59.5 intact and things improve after that.
A lot of things happened since then. First, the ACA was created and that has kept my HI costs under control, unlike the skyrocketing premiums I faced in 2010 and 2011. Then, the markets improved a lot starting in 2009. On the downside, the big bond fund's returns have shrunk considerable although I had added many more shares of it to offset the return. I never had to tap into principal, which is nice, so my buffer has only grown.