When placing pillars (Ardent Mages), the net FIDSI output should be shown (similar to building a district or selecting settler city position).  This is especially helpful when the radius of the pillars effects is 2.

Optionally/Additionally when placing in a settled region, the FIDSI of tiles exclusively worked by a city should also be visible and/or show the changes on the tiles themselves as you move the pillar around.

I love your idea