Hi Vivian! As you saw, LOD calculations can only summarize row-level fields. They don't work on fields that have already been aggregated (including table calculations). However, you can layer table calculations on top of each other, though this becomes quite complicated quite quickly.
Here, you'll only need two layers. The first layer is the one you have already, [% Change]. You can use another table calculation to compare that result to your threshold. Maybe something like:
IIF([% Change] > 5, "Fraud", "Not Fraud")
Although this doesn't have any table calculation functions in it, it's still a table calc because it refers to a table calc. In this case, you're lucky that both layers of table calcs will compute along the same dimension--either Table Across (I imagine) or along Customer ID. In other situations, though, you may have to edit each layer separately (using the drop-down in the table calc editor) to assign different compute dimensions.