Thanks for your help. This is what I'd been doing but I'd like it to behave as it would in reality. If I'd specced a wall to have a 30mm frame and a 20mm shim, the opening should be 100mm overall larger than the door size. Additionally, why the trim should have a bearing on the opening width is a mystery?
THe wall framer seems to have a few bugs. We've been perservering with it for a number of weeks now and although it helps initially set up the drawing, every single frame has to be checked very closely for compliance with the plan and a high number altered to work properly. It would also be useful to be able to specify the maximum length for a single panel. We currently chop walls to create additional frames. I don't know if this is your experience or not but would be interested to hear your views.