Distance from one language to another

Hello!
Do you know a way to measure how far a language is from another?
And I have another question similar to the previous one: do you know
a way to quantify the "rate of evolution" from a language from a
language it is derived from.
I remember reading once that French was 44% different from Latin,
whereas Italian was just 17% or something in this area.
Thanx for your answers!