Unable to load "Tile Editor (Newer) (Sunsoft)" - disk capacity mismatch?

[TakuikaNinja]

Trying to load this development tool on the FDSKey results in error 25. The display shows the disk as being filled completely, despite the fact that all the file blocks fit well within a single-sided .fds file. I'm wondering if there's an inaccuracy regarding the gap size between files (mainly block types 3 & 4). Editing the file amount to 3 (the minimum required to boot the tool) works fine for now but it's quite annoying that the included graphics files cannot be loaded at the moment.

The Sunsoft Dev Disks which contain the program in question will not be linked here. The archived disks aren't hard to find.