Good to have. I recently worked out the formula myself on a calculator, but I didn't write it down or put it into a spreadsheet. This saves me from having to do that
In any case, the greatest return on effort seems to be three D&F's for my vehicles. The margin rapidly diminishes thereafter. Since I've already done two D&F's on two of vehicles, I'll henceforth do a D&F annually. The fluid is already much cleaner than it had been, so the annual will be a little refresher for minimal effort.
If constantly renewed fluid were more desirable, the better solution would be one wholesale change of fluid, and then a D&F at some user-specified interval, such as each OCI or annually.