I guess we did it all wrong. Never actually documented spending until last year (5 years post ER). When w*rking and living in the Midwest, I estimated spending as the difference between what I (and DW) earned and what we saved. Crude, but effective when you are still w*rking. Since the savings were anywhere from 25% to 40% most years, I knew there was some "give" in the budget if we ever wanted to change our lifestyle.
BUT, our real ER goal was to move to Paradise. Although we had visited many times and even had our living quarters paid for here, we found it difficult to plan anything approaching a budget. Back-of-the envelope was about as good as we were going to get. My plan was to "over-plan" in case we got it wrong (or lost the envelope

).
Update: Our spending last year (discussed in another thread) turned out to be considerably higher than the old envelope would have suggested (well, full disclosure, we DID actually lose the envelope!) We spent more than I ever earned - probably more than I and DW earned together some years (especially when she was FT stay-at-home-with-kids and PT empl*yed).
The good news is that the "over-plan" plan is working. Quite simply, the plan was to save enough to more than meet the "envelope" budget, plus have backups to our backups, e.g., DW takes her small SS, but I'm waiting until 70 or until the envelope method starts to fail. Additionally, we have never exceeded a 3% SWR - and the stash is now significantly larger than it was at ER.
I DO NOT recommend this to anyone else. It was just a matter of practicality on our parts. Without living the lifestyle, it is difficult to budget effectively - nor was budgeting our usual MO. We also encountered several "surprises" post-move. Our medical has been twice the cost we predicted and OUR inflation has far exceeded the "official" rate.
Still, it has gone according to "plan", so, even though it's not for everyone, it works for us. YMMV