Assume you're viewing a file using texx2, and you've set the magnification level to 500, meaning that you're viewing your document at 1/2 the size that would be actually sent to your 300dpi printer (that is, you're viewing it at 150dpi). If your file calls for font ``cmr10'' scaled 1000 at 300dpi, texx2 will look for the font ``cmr10.150''. The suffix is compute by (dpi * scaled * mag ) / ( 1000 * 1000 ). You can generate this fonts using ``mftobdf''. For this example, you would execute ``mftobdf -scaled 1000 -mag 500 cmr10''. This looks for the font ``cmr10.300'' (the default resolution is 300); this would then be shrunk by 50% to produce ``cmr10.150.bdf'', the output file. mftobdf can only magnify things by 500, 333, 250, 200, ..., 1000/integer, because it does the shrinking in a very silly way. You could also generate this font by using Metafont to build a font ``cmr10.150'' and then saying ``mftobdf -scaled 500 cmr10,'' the result to texx2 are the same; it doesn't care how you produce the font.
texx2 was designed to work with mftobdf and buildfonts (a shell script to automate the installation of the X11 fonts). When texx2 can't find a font, you will get a message in the dialog box, e.g, ``Can't find font: -scaled 1095 -mag 500 cmr11.'' Simply cut the text ``-scaled 1095 -mag 500 cmr11'' from the dialog box and paste it in as the arguments to a buildfonts command.
The texx2 distribution also includes a file called build-initial-fonts that purports to build many of your favorite cmr fonts automatically. You should probably look at this an buildfonts if you want to automate the procedure. If you come up with a good solution, please inform the author.