Instructions:
Pick a unit (to be used for all three dimensions). Pick a paper size (you can mix and match paper and units). Enter the two diameters of your cone section. One of them can be 0. It does not matter which one is the big one: the program will do the right thing. Enter the height and click calculate. Depending on your browser setup, you will either see your template right away or you will be prompted to open it or save it. Some browser might warn you that this type of file (pdf) can harm your computer: don't worry I have no idea how to generate such a file!
Once you have the file, printing it at 100% can be a challenge on some systems (I'm looking at you, Mac OS X). Settings for "no scaling" or "100%" or "full size" may be scattered around in more than one dialog box... The printed dimensions should be right within 1/72 of an inch (that's the internal unit used by pdf).