To my greatest regret it seems that we have introduced this problem with the Q1 2008 release. It is already logged in our bug tracking system and will be fixed in the upcoming service pack due in a couple of weeks.
Fortunately there is an easy way to workaround this problem in your specific case. This expression should produce the same result:
|=100 - Fields.Included * 100/Fields.Total
In case the things are more complicated you still can use an user defined function
to perform the calculations.
Please accept our apologies for the temporary inconvenience. I have updated your account with some Telerik points.
the Telerik team