Here's my way of thinking:

You should have calculated the pps each device is using and have gotten the max allowed devices out of this calculations (25000 / pps_device_using = max_devices).

Your calculation of 100mbps/4320 ends up with a theoretical 23148 max devices if all devices use 4320 bits per second constantly. This is the number that decreases when you add more bytes, which is logical. Less max devices when using more bytes is not that hard to imagine.

While I can't finish your answer (because I don't know exactly how many pps each device is using), I can only guess that 45 devices with 4320 bits of data per second is probably never going overrun your NIC. My loose estimate is that your current design would hold up with 4000 devices.