Dan - please start a new thread for this and re-post your column definition - it's really hard to follow issues in these big threads.
What section are you creating your column in?
It doesn't always work to base one column on another, but let's get this separated out to another thread and get as much detail as we can, and I'll investigate further.
Please give an example of an expression which doesn't validate and I'll see if I can work out why.