Can I use the roll20 files for foundry vtt?
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So I got this working. You can just use the Roll20 version of the tiles, and in the Tile Controls > Tile Browser, just make sure the 'Asset Grid Size' is set to the width of my tiles (484px). Then they just slot into place.
Does this solve your problem? I'm happy to export them differently if it is helpful, but this seems to work.