1 Litre = 1000 ml so the range is 300 ml to 6000 ml per minute
at 1000 ml you get 5880 pulses but is that over a minute otherwise over a second its 5880/60 = 98 Hz square wave which has a period of 1/98 = 10.2 milli seconds
for 1 ml you calculate 5880/1000 = 5.88/ 60 = 0.098 Hz with a period of 10.2 seconds
While you could use the pulseIn to measure time of a pulse you might be better off using a frequency counter library. What you should do is a web search for Arduino frequency counter and you will find a library file with examples on using it so using above calculations a flow of 1000 ml/min results in a frequency of 98 Hz unless the 5880 is for a second then it would be 5.88 KHz for 1000 ml