How calculate font sizes on pads?

Dear ROOT,ers
I have following problem - I would like to draw histograms without margins on pad. First pad not only store histogram but also empty space for histogram’s description.
The problem is that size of labels and offsets are different and I don’t know how to scale them. Offsets on second pads are larger and fonts are smaller - so those two values are not proportional to pad size.
Does anybody know how to scale those sizes?
problem.C (851 Bytes)

Use the font size in Pixel. Font precision 3.