Global variables are file specific.
Setting the global in file A doesn't do anything for the data file - where the calc exists.
Ahh! Thanks Bruce. I had focussed on the Current Session part of their scope and missed that. I'll have to rethink.
Note that scripts in your UI file coud pass variables to scripts in the data file; and declare the variables there.
But normally one of the points of separation is to eliminate or at least greatly reduce script calls to the data file.
Thanks for your help Bruce. I've addressed the issue by posting the existing field content into a temporary global field in File B. That seems like the minimum maintenance solution.