We need to know specifics - what is the calculation, what is the conditional part. But here is an example of using a layout-level variable, if this is what you mean by global variable on a layout. There is no need for a calculation at all with vs. 11.
I used the Create Text icon to place a text area on the layout. Then Inserted a Merge Variable into that text area. This process requires a global variable which I defined in a script and it is the result of a calculation using a couple of fields on the layout. Works just fine. When I move to a different record, the "calculated field" displays the new result for that record. As expected. Now I want to apply conditional formating to that field (eg red text if the value is negative) and I can't get that to work. I have used conditional formating on other fields with no problem. Is conditional formating not allowed on global variables or merged variables?
This Merge Variable scheme is the only way I have found to do a calculation using fields in a layout and show the result on the layout without creating additional fields in the underlying tables.
I don't know why you think a global variable is required nor script. Script triggers and layout-level variables handle it all. Can you upload your file somewhere and provide a link?