From Luke Johnson at What Revit Wants
In this workaround, a Calculated Value is added to be used as a 'constant' (it doesn't really matter what number is used here). Then, another Percentage calculated value is created against that constant...
Count to grand total ratio in a schedule
Here is how you can do it.
1. Define a formula calculated value: Name - Workaround, Type - Number, Formula - 1.0; Hide the column showing this value
2. Define a percentage calculated value for Workaround.Leonid Raiz
Count to grand total ratio in a schedule - AUGI