Many experts recommend that we carefully monitor the calcidiol levels in our blood and adjust the dose. However, we don't really know what the ideal level should be. If my hypothesis is correct, then we should simply make sure the total dose we get from the Sun and from supplements is at a level consistent with being outside during summer all day long. This amounts to a dose of the order of 10,000 IU per day.
Note that dynamical equilibrium between production and destruction of vitamin D in the skin is reached on time scales of roughly a half an hour to an hour. This means that we would reliable get this maximum dose, if we would be outside with most of the skin exposed to the Sun for an hour or longer. Even if the sky is overcast, we would still get this dose, if we are outside for several hours. The dose at which dynamic equilibrium is reached doesn't change if the flux of UV radiation becomes less, what changes is the time needed to reach equilibrium.
This implies that the vitamin D dose has a natural constant value which is set at the maximum dose we can theoretically get. What is "theoretical" for us today, was quite normal for our ancestors. The calcidiol level cannot be considered to be a constant, as this depends on how fast your body uses vitamin D. If your body is fighting an infection, the immune system will use vitamin D at a faster rate than normal. The natural dose would only start to become less than the maximum dose, if the weather were so bad that your Stone Age ancestors would not get outside for long periods of time or if they would have to wear thick clothes covering most of their bodies.
My recommendation is therefore to take 10,000 IU per day during those parts of the year that you don't get any vitamin D from the Sun. Note that the UV index has to be 3 or higher for significant amounts of vitamin D to be produced in the skin in a period of an hour or less.
Of course, if your vitamin D levels were too low to begin with , you should follow the recommended treatment, which does involve monitoring of calcidiol levels. But once your levels are in the normal range, you should not use a low dose of 1,000 or 2,000 IU per day but instead make sure you get on average 10,000 IU per day from the Sun plus supplements.