250 milligrammes is the same as 0.0525 teaspoons of dry substances including salt, sugar, and spices. A gramme, or 1,000 milligrammes, is approximately 0.21 teaspoons. Because 250 milligrammes is a quarter of a kilogramme, multiplying 0.21 by four becomes 0.0525.
This is a conversion from mass to volume. Although such conversions are not extremely accurate, the variations are usually considered insignificant enough to ignore in common domestic use, such as cooking. At the same volume, different ingredients have varied mass values, so a teaspoon of one ingredient may be heavier than a teaspoon of another.
A chef will require a vast database of ingredients, preferably at varied temperature points, to produce an exact conversion. This is required because many materials expand as the temperature rises but their mass remains constant, resulting in erroneous conversions.