Thanks for all the responses. My takeaway is that a lot of different methods can work depending on one's budget, income sources, and preferences. Since I've been used to a paycheck for so long, I think I'll do monthly withdrawals based on average "recurring" spending, and then supplement with larger withdrawals to fund big ticket occasional purchases.
There's a couple of problems with this. I know about these because I started off the way you said.
* You still have to stay, on average, within your planned SWR, 4% or whatever. If you take sporatic large draws, how will you know that you are still on track?
* When a big-ticket item comes early in the time-period rather than later, where does the money come from? If you are doing "paycheck-like" draws, then you have to effectively borrow the money and then pay it back by taking smaller draws afterwards. How do you know when it balances out?
* Let's say your 4% SWR is $5000 and your average recurring expenses are $3500. Treating this like a paycheck, what do you do with the remaining $1500? Probably save it, maybe to a savings account. But then you say, "Hold on, when I was working I'd just invest (some of) that money in my investment account." So you draw $5000, spend $3500, and put $1500 back in. After a month or two you realize that taking out $1500 and putting it right back in is kinda silly.
* There are many advantages to treating your SWR like a paycheck. But I never had a paycheck where I could only take half-pay now and the rest of it later whenever I wanted to. Or a paycheck where I could take triple-pay now and then half-pay for the next 6 months.
* Every once in a while your expenses will line up such that you have enough cash-on-hand to handle them without withdrawing ANY money. So you leave the $3500 in, and make a note that in a month or two you can spend that $3500. Effectively, you will spend the April draw in May. Or in July and August. Or in July, September and December.
* When you are FIRE, you can do things you never could before. Like: Get a last-minute great deal on a Galapagos cruise/tour. $30,000 for the two of you, compared to the regular price of $50,000. Or pay for your son's unexpected wedding: $10,000. Or your daughter's unexpected $15,000 divorce legal bill.
You've got plenty of money, just not the lump sum. So you take out the large draw now and pay it back by taking smaller draws for the next year or two.
After awhile it becomes obvious that you have to keep track, both to see how you are doing compared to your target SWR, and to know when you have finished the paying back the large one-time expense, and to know if you have saved enough (by lower draws) to pay for a large expense.
* When your wife looks at the portfolio statement and says, "We have $1 million dollars, so why can't I buy this $3000 diamond necklace?" You'd better have an answer!
Or maybe it's you asking why you can't buy this $5000 bass boat.
If you are keeping track, you just look at the spreadsheet and see how much over/under you are.