Any TFS gurus? Need help with 'Completed Work'.

I've posted this over on the TFS MSDN forums, but I thought I'd ask here as well as some devs frequent this forum more than the MSDN ones.


We're using a customized version of MSF for Agile on our TFS server.
There's something I'm obviously missing though...

When setting up a task, we give it an estimated number of hours, which is mapped to 'Microsoft.VSTS.Scheduling.BaselineWork'.
When a developer has worked on that task, he increases the 'Completed Work (hours)' field which is mapped to 'Microsoft.VSTS.Scheduling.CompletedWork'. However, he/she must manually remember to decrease the 'Remaining work (hours)' field.

I don't get it, why isn't the remaining work hours field simply a calculation of Baseline - Completed?
Does anyone know a way to acheive this - ie, whenever the 'completed hours' field is changed, calculate and enter the value of Remaining hours? If that's possible to do, then I can mark that field as read only.
I'm thinking it may be possible to do by using the Work Item Editor and modifying the 'workflow' steps? Unfortunately, this is the one area of the WIT editor that I'm still shaky on.

Many thanks.
Richard.