The nice thing about estimated tax payments is that they're only an estimate. You just need to guess high enough not to be penalized -- which, with the "safe harbor" rule, usually means that just sending a quarter of last year's taxes each quarter is more than good enough, if your taxable income/gains are anywhere close to what you pulled in last year.
Yes, if you're pulling in less money this year, that would be overkill, and you'd lose a bit of profit that you might have made if the money had stayed in your pocket rather than being on short-term zero-interest loan to the government. But my perception is that for many of us that won't be enough money to make much real difference.
Similarly, the penalty for undershooting your estimated taxes may not be large enough to make a huge amount of difference if your total tax bill isn't very large.
So my own take, for my individual taxes, is that it isn't worth my spending a lot of time on trying to get perfect. Glance at the income for the quarter, take a very rough guess at what taxes will be owed, and call it good enough.
If you've got much higher income than I -- if you're running a business rather than just living off your investments, or if you've cashed out a great deal this year for a major purchase, or something like that -- more precision is probably justified. But for dividends on total investments of less than (handwaving) US$5M, I suspect it isn't.
