There is a way I found more convenient
Create a parameter name diameter, as a real number, use the relation toolbox to equal d1=diameter
Then when you edit the family table, add new colums, select parameters insted of dimension, then select the param diameter and load it into the family table.
The other way works, but if you need other dimension to use the diameter value as well, you can't name two dimensions with a same name (in this case diameter). In case if you family table grows