I almost always wait until the end of my vacation window to book our PSs all at once. I'm always able to get exactly what I want....restaurant and time. I think that there are relatively few of us that are such compulsive planners. In the last couple of years as life becomes more complicated with our grown children and their plans, I often end up tweaking my PSs and still have no trouble.
Of course, there are the well known exceptions....CRT breakfast (maybe EPCOT Princess breakfast), Christmas Eve and Day, NYE, probably Thanksgiving, V&A Chef's Table. If you're trying for any of those, I would do it on the first day you're able. Otherwise, I'd save those transatlantic calls.