Firstly a little more detail about your application would help return a better answer. Second, generally speaking in natural systems it is rare to find linear relationships and so you cannot rely on having two points make a line. You need more. But most importantly you need points "in the middle". If you are lucky the relationship between the remote sensing output and the feature of interest will be a modest curve and the midpoint of that curve will be easier to define if you have some or many points near the middle.

With regard professionals... I say that all they know is how to cut the corners, so don't do what they do! Use best practice and find a suitable statistical test which tells you when you have gathered enough data. Plotting stuff up on the fly is a good start.

My contribution at least

Firstly a little more detail about your application would help return a better answer. Second, generally speaking in natural systems it is rare to find linear relationships and so you cannot rely on having two points make a line. You need more. But most importantly you need points "in the middle". If you are lucky the relationship between the remote sensing output and the feature of interest will be a modest curve and the midpoint of that curve will be easier to define if you have some or many points near the middle.

With regard professionals... I say that all they know is how to cut the corners, so don't do what they do! Use best practice and find a suitable statistical test which tells you when you have gathered enough data. Plotting stuff up on the fly is a good start.