It is probably getting muddled because the condition you have created works on row level - as all calculations do. So you need a step in between:
Calculation 1 : IIF([PRODUCT_ID] = 'X', 1,0)
Calculation 2: sum up calculation 1 for transations and look for values 1 and up.
Hope that helps,
Thanks Dana but I don't follow you.
Found the solution in the on demand video. My formula looks like....