Monitoring a fan with 2800 RPM with a Siemens 27-226 PLC
I would add some head-room. Most likely the fan's speed will vary by a few rpm (or a few hundred rpm if it's not too critical what the actual speed is).

Another potential method is to use an "inverse counting" method for calculating the fan rpm.

If your application can't tolerate the fan being off for a minute before you realize something is wrong, or if you want to be able to distinguish between a fan which oscillates between 10000 rpm for a bit and then off for a bit, you can instead trigger an external "high speed" counter to start/stop whenever you receive a pulse.

The frequency of the fan is then f/count (or f/(2*count) for 2 pulses/revolution), where f is the high speed counter frequency. With a ~1kHz pulse rate this should give you up-to-date information on the actual rpm of the fan every ~1ms as opposed to every second, or worse every minute. You also get much better resolution on the actual frequency.

However, with a 30KHz high speed timer your expected count between pulses is ~30, so this method may not be the best unless you can get a faster high-speed timer.