Converting cents to ratios only makes even a bit of sense if you have some preexisting music in some temperament, which you then want to approximate in just intonation. But even that isn't really something that would be served by calculation from cents and not simply determining yourself what ratios you want (because, at base, deciding which ratio you want to have is entirely arbitrary). Now, if you have some closed scale, it's simply a matter of figuring out the cents values for the notes in the scale, and then rounding your preexisting notes to the nearest one.
But if you are using any kind of temperament at all, you're dealing at base with irrational numbers, and no computation is going to provide meaningful results unless you specify beforehand what your limits are (in which case you're specifying a closed scale, and it's thus much simpler to do the calculations in the other direction). For example, here are all the ratios in a very basic scale that fall within a quartertone of 600 cents: 11/8, 18/13, 7/5, 17/12, 10/7, 23/16, 13/9, and 16/11. Which one of those counts as a "tritone" for your scale depends almost entirely on how simple you want to keep the ratios (and if simplicity isn't necessarily your goal, there's also 352/256, 89/64, 179/128, 359/256, 181/128, 367/256, and 373/256; all of those are pure overtones).
Saying you want to convert from cents to ratios is a self-contradictory proposition. The degree of accuracy you want depends on an a priori determination of the limits of your scale, in which case you're working from ratios regardless.
Meanwhile, I'm teaching myself fontforge. And here I thought it was commercial software, and I already had it installed.
Cheers,
A