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					Originally Posted by  kovidgoyal
					 
				 
				Well you should start thinking of them in units of length, that way if you move your settings to a computer with a monitor with different pixel density, they will still appear the same size. 
			
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 No need for me to wait to get another computer I have three monitors available on the one I am using.  If I were to run Calibre on the 22" Lenovo I would want to see more covers than I would on the 30" Dell - the Lenovo is 3840x2400 pixels, the Dell is 2560x1600pixels.   If I run Calibre on the 24" Samsung at 1920x1200 then I'd probably want to see about the same as I would on the Dell - the Samsung normally being a lot closer than is the Dell.   
Viewing comfort is at least as much a function of pixel density, the dot pitch and the distance between the eye and screen, as it is on screen size - I would argue more so.
I used an image editor to work it out, easier than doing the sums in my head - now I have a 12x5 grid on the 24" 1920x1200 Samsung.  Next time I run Calibre on the other monitors I'll work out a good size for them and write it down - somewhere 
I worked on hi-end imaging system for a decade or so, back then we only thought in units of length when we printed images for situation room briefings.  AFAIK camera's and scanners still work in pixels - even the ones on fiddle fones - never mind. 
BR