There is a natural tendency when learning Sight-Size to measure first. It seems easier that way – you get the measurement, plot the point and so on. But if you do this, you are drastically limiting the opportunity to train your eye to see. A better way is to habituate yourself to the guess and check.
Do you recall learning how to read? When you came across a word that you did not know, did your teacher simply tell you what it was, or did she first tell you to sound it out? I suspect the latter. The process taught you what the word sounded like and through your own voice you learned the word. If she had simply said the word to you instead, your learning would have been limited. Later on, when you came across the same word, you would likely have needed to ask once again. Unless, of course, she was not there when you read the word the second time.
Similar scenarios play out in ateliers, academies and art schools everyday.
The unknown word is a shape or placement of something which you are drawing.
The teacher is your plumb line, mirror, etc.
What do you do?
You should guess first, then check. That way, over time and as your eye improves, your guessing will turn into knowing and your checking into confirmation.
This concept is as true for those who isolate value and color. Instead of shading or mixing to the chip, guess first, then chip.