I need to check my vitamin D levels regularly for my disease. It doesn't seem possible to measure at home, but measuring calcium in the blood may be more achievable since they are directly correlated. But then how can I do it?
You should not use calcium as an indirect means to evaluate your vitamin D status. While both are metabolically connected different mechanisms are involved meaning that there is not a one directional correlation across time, and many other things, unrelated to vitamin D, affect blood calcium levels as well.
In the short run, all else kept equal (which is a culprit), a higher vitamin D status will lead to a greater portion of a dietary dose of calcium to be absorbed. However, this will also lead to increased bone formation, and soft tissue mineralization, which has the opposite effect on blood calcium levels in the short to medium term.
With time and resources you could probably develop a model for a very controlled process to measure your vitamin D status through your blood calcium levels, but it wouldn't be practical for people to use in general, and it would also cost you personally much more time and resources than just getting a vitamin D lab test every 3 months for the next couple of years.
While blood levels of vitamin D may fluctuate in the short term, your overall vitamin D status fluctuates slowly (which is due to the fact that your body fat acts as a storage buffer for it). That and the fact that is has become a very routine test makes it affordable to monitor via routine lab testing.
If you develop a proper habit to expose a greater part of your bare skin to sunlight once every 7-10 days, and have a stable diet, you can just use initial testing to confirm the effectiveness of that habit and then save even more money by not testing for your vitamin D levels at all.
Your body'd capacity to efficiently produce vitamin D will slightly decrease over time as you get older, but that happens only very slowly, so you can consider the execution of whatever habit you have developed to be a good way to predict a good vitamin D status.