if you want to be super accurate about this, the math should be done on the raw PCM data chunk only, without the wav header. You probably have slightly more free space in the Rytm than your catalog shows.
Headers for wav files are usually small, but can vary in size quite a bit (metadata, memory-page padding etc). One could create a pretty large wav file which contains no or very little data, and it would check out legit in most software...
when sending a sample via MIDI, only the PCM data inside the file is transferred, and I suppose the Rytm also stores just that plain data on the +drive, plus just a few extra bytes for the name, length, ...
for a 16bit wav @48kHz, the PCM size in bytes is:
length_in_seconds * 48000 * 2 * num_channels.
it's easiest and most accurate to use some software which shows the number of sample points/frames and then:
num_frames * 2 * num_channels.