realized I could solve the minecraft problem of
"I haphazardly partially-filled a body of water top-down, and, not knowing what's source blocks and what's just downward flow, I cannot know whether I'm done"
by simply using litematica to make schematics of source blocks to compare against
after a bunch of trial and error I arrived at :
can't have "overlayReducedInnerSides" set to false or else it produces dozens of transparent layers that combine to form a solid color that gives you approximately zero information
kinda don't want to even use all layers render mode at all
single layer render mode doesn't have a top side on missing water blocks unless it's the top of the water in the schematic
therefore my optimal use method is to move a 1-tall schematic of a flat plane of water upward through my 60-high box of salmon and drowned