1. Create two elements and select them.
2. Use Alignment button from the top toolbar and pick Space Horizontally or Space Vertically.
3. Uncheck Auto distribute and fill in 0,75 mm and hit tabulator or enter key.
Result: The value in the input field jumps to 1 mm.
In fact the distance is correct, but not the shown value. To avoid the next question: I have 3 decimal places set as preference for millimetre.