Don’t forget the miles (and even percentage) is just an estimate. The estimate is more accurate if you often fully charge and fully discharge, because then it knows its boundaries.
Don’t worry about it. You’ll lose a few percent in year one, then about a percent a year after that. In 5 years there’ll be twice as many superchargers
And the Estimate is not a hard number. In the desert, the battery operates in a warm condition. In the winter, the battery may operate in the bottom of its heated/cooled range. The Estimate will be figured at battery temp, that could vary fill by fill. Losing 1/223 miles is in the noise level of estimates. I would bet that setting your charge to 90% has some wiggle room in it - Could it be 89.5% one time and 90.5% the next?
I remember one fellow saying "measure with a micrometer, mark it with a piece of chalk, cut it with an ax".
For whatever it's worth, I have a Model S 90D (June 2016) with ~23,000 miles on it. The rated range was 294 miles at delivery, and the last time I charged to 100%, it still read 294 miles. I charge daily to 70% and it reports anywhere from 203 to 207 miles (seems to vary with temperature). I only charge to 100% once every few months. I've supercharged about 50 times or so. I often wonder if the "rated miles" readout on the car truly reflects battery degradation. I've read several times that the 90KWh batteries may suffer from higher than normal degradation, but I guess I just got lucky?
My experience with our Model S75D, March 2017 build, is almost identical to yours. Charged to 100% this morning and got 249 miles, when it had sat at 250 miles for a few months. Charging to 90% yesterday netted me only 224 miles.
Out of necessity, I charge to 100% at home weekly, and anywhere from 90% to 100% at Supercharger, also weekly.
We seem to have lost 10 miles of rated range in fourteen months of ownership. I was hoping we would get a few miles back as the weather warms back in Southern California.
Still, there doesn’t seem anything I can do about it, and fretting about it doesn’t seem to help.
There’s a saying I learned from a Mexican colleague of mine many years ago - “If there’s a way to fix, why worry about it? If there isn’t a way to fix it, why worry about it?”