I am a webmaster. Excerpt from a conversation with a dear client (dama) to whom I make a website:
...
Q: What kind of computer do you have?
The MacBook Pro!
I : Oh you! Iron or plastic?
K is iron! Grey is like that. What about CHO?
Q: We left the topic. On the site between the pictures only 4 mm, and they themselves are 9.1 x 6.4 cm. That is a lot!! I do not like that!
I: (I understand that he sits and watches the line to the monitor) Now we’re back. It is very important to know! What is the screen length? (I’m getting my wife’s Macbook from the shelf)
See also: 33,6
I: From the edge to the edge of the cover or from the edges of the black line? (I quickly consider and attach the line to the MacBook monitor - approximately 33.6 - the length of the cover)
K: From the end!
I: It is great! (It is just... the answer.) Let’s continue to make the site. So, how many millimeters should you make between the images?
Q: Let’s try 12.2 It will be good.

Further creation of the site took place with a line in hand and two compasses (my Aymak 27 and my wife's MacBook). Two days later, when my brain stopped blinking, I was already able to navigate freely in this coordinate system.