rounding number problem
I do not work much in metric units, and almost never with meters as the base linear unit, but in some brief tests in setting up a project, the precision that is applied to the Floor-to-Floor Height and the Floor Elevations in the Levels worksheet seemed to be controlled by the precision settings for linear units in the template file assigned to the project. I was using a common model/view template, and that template's linear units setting appeared to control the precision.
Accepted solution
With your project open, in the Project Navigator, on the Project tab, select the Edit Project tool at the right side of the Current Project header.
In the Project Properties dialog, verify that the BASIC category is expanded. Under the BASIC category, expand the Templates sub-category if it is not already expanded. Make note of the template file(s) being used (both the file name and the path to it). In my tests, I had Use Common Model/View Templates set to Yes, so the only template I modified was the one assigned to Common Model/View Template. In the image below, this is the Aec Model (Metric Meter Stb).dwt*** file.
Close the Project Properties dialog, and close the current Project (one way - in Project Navigator, on the Project tab, select the Close Current Project tool at the bottom of the tab, second tool from the left. Use the OPEN command, and, in the Select File dialog, set the Files of type drop-down list to Drawing Template (*.dwt). Navigate to the folder where the template file noted above resides and open it. Use the UNITS command to open the Drawing Setup dialog with the Units tab current. Verify that the Drawing Units are set to Meters. In the Length area, what is set for Precision? From what you posted, it is likely 0. Use the drop-down to select a different precision. If you want millimeter precision with meters as the base unit, choose 0.000. Save and close the template file.
If you use the same template for all Model/View files, you are done. Otherwise, you may need to repeat the above for each template file you use.
Use Project Browser to re-set your project current. Are the level elevations on the Project tab of Project Navigator now showing the precision you set in the template file? (Note, if you previously entered 3.2 and got 3, it will likely still show 3, but with the additional precision you selected (3.000 if you chose 0.000 for the precision). You should now be able to edit the elevations and have decimal places up to the amount you specified for the precision be saved.
*** - I created the Aec Model (Metric Meter Stb).dwt template file by copying the Aec Model (Metric Stb).dwt that came with my installation (United States English content) and changing the linear unit from millimeters to meters in the Drawing Setup dialog, on the Units tab. The metric content files I had were all based on the linear unit being millimeters.