If I understand the need, it is easier than you make it out to be. There are a few levels of 'scope' for a variable within the code for a project. For your case, it is enough to Dim the variable a the top of one standard module in order to make it visible to the whole project so that you can use it from anywhere. However, this is seldom a great idea. Rather than repeat the reasons why, look at http://wiki.c2.com/?GlobalVariablesAreBad IMO, one of the most potentially dangerous reasons seems to be missing there: if the app errors, global variables can revert back to default values or data types. Depending on the need, a custom class object or function might be more advisable. You can expose the function globally but use the value it returns locally.
If you want me to expound a bit on variable scope, let me know.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.