Jaco, I wrote a dosing calculator a while back based on Roark's calculator. It does liters, imperial gallons, and american gallons. The doses spit out in grams, teaspoons, etc.
Here is a direct link to the web page. It is free and open source. However, it requires Windows since it is a Visual Basic program.
http://my.starstream.net/sarge/KoiCalc.html
(If you start at
http://my.starstream.net/sarge/ you can browse the rest with frames enabled, but I didn't build the page to correct itself if the navigation frame is there or not so it'll open new windows for everything.)