When I say fair value, it's what I consider to be fair value. As an investor you always have to ultimately make those decisions for yourself, or you have to defer to another person's judgment on the matter (whether a talking head on TV, or pump & dumpers on Reddit, or newsletters, etc). I'm not basing that on something some guy put into a book 70 years ago about how to value a stock, even if some textbook'ish knowledge can be worth learning to use as you go about coming up with your own valuing formulation (as in the case of Ben Graham). It's based on my past ~26 years of experience with stocks and what I look for in investments. You'll find with experience as an investor, if you're self-educating and or managing some or all of your own investing, you'll come up with your own tests for investments, your own way of valuing what you're buying & selling (or you should anyway). You can take pieces here and there from others and assemble it based on how you like to invest, inevitably over a lifetime it no doubt becomes an amalgam from what you learn.
So for example if I think the fair value for Coca Cola (KO) is 30% to 50% lower than where it's at today, that's not based on a textbook valuation approach. I base it on what I'm willing to pay for growth, and Coca Cola is a pathetic non-growth machine (not to mention a giant sugar liability). I look at Coke's financials and, with some understanding of their business, I ask: what am I willing to pay for zero or negative growth across time? China's boom has come and gone and Coke's growth - as a global business - has recently been stagnant, mediocre, so what are their prospects going forward? I don't like that picture at all. I might be willing to pay somewhere between 8 to 15 times earnings for zero growth (depending on context; I might pay less for a financial firm than a tech firm, and so on), if there is something I like about a company. Coke's multiple is closer to 27-33 lately. Why would anybody ever pay 30 times earnings for zero growth and bad prospects for growth? Coke is a very easy fair value calculation as far as my personal judgment is concerned, their persistent growth problems make that a super fast decision. I'll look elsewhere. McDonald's is in a similar boat as Coke, it's a horrific value proposition, 30+ times earnings for a business with very little (or negative) growth. I might pay 12-15 times for MCD or KO, maybe. Personally I tend to really dislike companies with no growth or weak growth prospects going forward, it's a giant negative in the margin of safety calculation (growth is a first-aid kit for problems that inevitably crop up in a business over time, random messes, it applies a bit of a balm, helps as an offset in the value calculation; if you don't even have growth, inevitable problems are that much worse when they happen).
Fair value means I've looked at the stock in a way that I prefer to approach a stock and I've made a determination for myself, for my investment purposes, as to how much I think it should be worth. And I may come up with a few versions of that, one for an average market (with typical multiples), one for a slightly bubbly market; typically I disregard trying to come up with a value based on a mania, I'm not a buyer at that time in most cases. Those variations, models, are meant to inform myself as to the flex in my investment. If valuations merely go back to where they were in 2012 or 2016, how might my investment perform if its multiple is reset 1/3 lower? Will I get killed on the price I paid? It's modeling.
Interest rates will absolutely distort the context of deciding what something is worth, that falls into the variations, models, you build for different scenarios. The point of doing that is to check / prepare your position against a bad outcome. People claim that low interest rates will keep stocks inflated, so there's nothing to worry about; I like to point out that multiples were far lower at numerous points in the past decade when interest rates were at zero and we also had QE going on. How about if we just roll back to where multiples were in 2014 when rates were zero (and our economy was better positioned in 2014 than it is now, although our headline unemployment rate was similar)? If I were a buyer today I'd absolutely be running that simulation for myself whenever I buy.