You've bitten off a difficult problem. Assuming that the user hasn't overridden your style sheet, it's going to be nearly impossible to calculate absolutely correct numbers, but with a bit of manual work, you can get an approximation.

Start with some text of known width, and measure. 12345678901234567890123456789012345678901234567890 aaaaaaaaaaaaaaaaaaaa
...
iiiiiiiiiiiiiiiiiiii
...
MMMMMMMMMMMMMMMMMMMM

Then do a little measuring and division, and you can come up with a table of character width estimates.