The simplest way, if you have all the hardware to play with, is to set it up and see what figures you get. Inject your low scale signal into the analog input (e.g. for 4-20mA, inject 4mA, or for 0-10V, inject 0V), and see what number your analog input displays. That becomes your low scale input value. Then inject your full scale input to the card (e.g. for 4-20mA inject 20mA, for 0-10V inject 10V), and check the number your analog input is displaying. That becomes your high scale input value.
If you don't have the hardware, you'll have to find the manual for your analog input card and look for the raw data tables, which will allow you to calculate the low and high scale values for your chosen signal type.
Output low and high values are easy. Work out the range of values you want/need to measure, and then give yourself perhaps 20% headroom.
For example, if the maximum flow through your flow meter is 120L/min, then perhaps set your output low and high values to 0 and 150 respectively.
Of course, that's a guide only - to give you accurate information we really need to know more about the system.
