I am an Inkscape newbie and don't know much about computerized graphics in general, so be gentle

My problem is as follows. I am using Inkscape for scientific illustrations. The journal that I am writing to requires that the fonts will be 9-10 pts, and that the resolution will be 600 dpi. I sent them my illustrations as pdf's, and they told me that my fonts don't fit their demands. I know that Inkscape uses px and not pt for font size, so I used the following conversation:
7 pt = 9 px (this one is for superscripts)
9 pt = 12 px
10 pt = 13 px
than, when I save the file as a pdf I am defining the "resolution for rasterization" to be 600 dpi.
A friend checked my file and told me that my font sizes deviate by 3 to 8 percent from the sized I intended to get. I suspect that it has something to do with the raserization resolution.
I'd be more than happy to hear ideas.
Best,
Eyal