I have 256 compositions, all very similar and with the same black solid as the top layer. On that solid I have a basic expression that toggles the opacity on and off randomly, off being 10 times more likely than on.
If I wanted to change the likelihood from 10 to 20, I'd have to go and change the expression in all 256 compositions. Could I avoid that by setting that X number somewhere in my project outside of these compositions, and refer to it? I could then easily play with that number and affect all compositions.