After a few divisions of the interval by 2 , I got this result:

The face

(default (:family "Monaco" :background "black" :foreground "gray77" :height 100 ) )

provides exacly the same hight as

(font-lock-string-face (:family "Inconsolata" :foreground "lawn green" :height 1.20) )

So, if the height of the original font is 10 pixels, for the second font I need 120% of the initial height to preserve the height.

Can you explain me details about what happens ?