Hugo,
Each fontface is going to behave differently. I don't understand the hangup about not wanting to loop through the possible font sizes calling fontmetric() till you find the best fit. It's not like fontmetric() takes a ton of time to execute.
>Yes, I know about fontmetric, I think I can do an iteration that would work without using fontmetric, but I wanted to avoid the iteration, just for the sake of doing it without iteration :) (Apparently the word iteration is looping inside my brain!)