Tooq: Once you're FIRE'd, you can think of spending less than 4% as a savings rate (i.e. if you have 1MM, and spend 30k instead of 40k, you can think of it as a 25% savings rate--10k "saved" out of 40k potential income), or as a lower WR (a 3% WR). It doesn't matter what you call it, a 25% savings rate on 4% WR, or a 3% WR, in the real world, you spent 30k of your 1MM stache. It's six of one, half a dozen of the other.
Before FIRE, it's just your portfolio changing value, up and down, and it's gradually decreasing your WR to a sustainable level, as your stache hits higher and higher multiples of your expenses. But again, there you could think of it as adding to your portfolio.
The key to go back to though, is: What's the point of an SWR?
1) Sound Impressive? There are many ways you can jack it up, to sound more impressive, if that's your goal. Then I would use your method, among others.
2) Track progress? If its point is to track progress, I wouldn't use your method (because my stache growing on its own accord increases my savings rate over time, when really I'd want to use my actual earned income and expenses to see if I was earning more, and/or spending less, over time.. not just having my stache grow due to things beyond my control).
3) Calculate time to FIRE? If you're trying to do that, I wouldn't use your method, as it's expenses and earned income that counts in that calculation (stache growth matters, but in a "guess" way--not in a "increased savings rate" way).
4) Calculate the odds of your success in FIRE? If you're trying to do that, you won't count it as savings, you'll count it as a lower SWR, as that's what historical calculations are based on.
In other words, I don't see a use for the calculation where you count it as "savings." The first one, sure, but the next three, no.
You certainly CAN look at it that way (as I said, it's six of one, half dozen of the other--25% savings rate in FIRE or 3% WR instead of 4% WR), but it's just not really useful to do so.