How to close the screed measuring point, with what? And is it necessary?

We are talking about a minimal small cut in the floor. With tiles or parquet/laminate, you wouldn't need to close it at all. With very thin vinyl, it definitely makes sense, but you don't need extreme stability with such a tiny opening. The most important thing is that the surface is smooth in the end. In this respect, tile adhesive, homemade concrete, etc. are all suitable. It also doesn't make sense to buy a whole bag of screed concrete if in the end 10 milliliters disappear in a small gap.