How to calculate the font size?

[Magicloud Magiclouds <magicloud magiclouds gmail com>]

Hi, I am not quite familiar with the font thing. Now I have a simple
request, I do not know how to meet it:
Say I have a background (i.e. a cairo surface) which size was not
certain when the function was made, I have to write a "0" at (1%, 3%)
with the size of 5%, all % means the size of the actual background.
How to do it? Thanks.
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