Currently, a Canvas KPI Card only accepts a specific cube cell as data source (with the ability to multi-select some dimension elements and then use average, min, or max, like in a View). This implies that all data to be shown on a KPI Card must be present in a cube (cell).
This does force a level of centralization for data (avoiding crucial calculations to be done only in some report), i.e. "single source of truth", but it also limits flexibility a bit; in current web reports, side-calculations on top of cube data are quite common.
Maybe consider allowing a second option to KPI Card source data ), "Expression" - input would be a custom (spreadsheet) formula, again like is already allowed for comboboxes or labels. If this option is used, the "Comparison" should then also allow a custom expression as source (in addition to the current "fixed value" and "data point" options).
