Plant Health Reading

My issues is when I am working with the plant health I don’t know where is the best place to put the two blue dots to get the best reading of each field. I keep reading that each crop will read different. Right now I have been test corn and beans, and I am looking into flying a few wineries. So what am I wanting to see on the slider menu so that I know each time I pull it up I am getting a good reading. I am using a magic pro and an inspire 1 so I have my algorithm set to the VARI