Project Unit Rounding Matters

Let's say for example that you need a metric dimension value in your family and it should be 6.35mm. Let's also say that the family's Project Units are set to zero decimal places. When you enter 6.35mm in the Family Types dialog Revit will round the value to 6.4mm.

When you change the family's units to show two decimal places the value that was stored in the parameter is still the single decimal value (6.4mm). Revit did not store the 6.35mm value but shows the rounded value instead, it stored the rounded value.

If you override the dimension style to show two decimal places (as shown above) but don't alter the Project Units of the family you can enter the correct value (in the dimension string) and the Family Types dialog will still report the rounded value. If you change the parameter value in the Family Types dialog it won't respect a two decimal entry and you'll see the dimension value return to a rounded value.

Short and sweet, if you really want a value like 6.35mm to be honored you need to set the family "Project Units" accordingly.

Fwiw, this is different from the project environment where entering a value that uses two decimal places is honored even if the project units and/or dimension properties do not.