a normal 3.5in floppy disk holds 2MB. That's not due to rounding.

The disk spins at 300RPM, thus 5 rotations per second. The data-rate is 1Mb/s... but that's MFM bits, two of which make a data bit, so 500Kb/s of data-bits. Thus, 100Kb fit on each track, or 12.5KB. There are 160 tracks: 80 cylinders, two sides... 2 mega bytes.

1.44MB for storage is due to formatting. Sector-headers, CRCs, and a few other things. Most computers are capable of different formatting, at the hardware level, but software doesn't usually support that by default. Thus, it's more than possible to cram more than 1.44MB of user-data on a disk, but less than 2MB, by using a different format (fewer and larger sectors means fewer bits wasted on headers, more for data), *without* using compression.

But all that's moot to this project.

This project replaces MFM-bits with PWM-nibbles. Each MFM-bit is 1us long (1Mb/s data rate), each data-bit, encoded in MFM, is 2us long.

Each PWM-nibble, as I've currently defined it, is 5us long: four bits of data gets stored in that space. 

Thus, in MFM, 5us would contain 2.5 data-bits, here it contains 4.

That's 3.2MB per disk, vs 2MB. Of course sector-headers can be equally-relevant in decreasing the capacity for user-data... but I see no reason why it would be more overhead than on an MFM disk. 

That'd make the user-data storage-capacity of a PWM-3.5in floppy 2.3MB vs. an MFM-floppy's 1.44MB.

Significant!