Mr Beer is designed to make 2 gallons of beer... regular 5 gallons uses about 3/4 cup of corn sugar... if you scaled it, you should use around 0.3 cups.. If you have a balance, that should be around 1.6 oz.
Note that amount of carbonation (sugar) is a little different between different style of beers.
One thing that I like to do is make a priming solution... Don't put sugar into bottle or mr beer directly. It is hard to get consistent carbonation this way.
Instead, boil 1 1/2 cup of water, add the priming sugar, boil for 10 minutes.. Cool down to room temp, ~70F. Slowly pour it into Mr Beer Keg... gentle stir without splashing.. (sanitize the stirrer)..
With priming solution, you will get a more consistent carbonation.
In the past, I have used 1/2 cup corn sugar for priming.. It turns out pretty good.
Last note... Make sure your beer is completely fermented.. otherwise, you are having too much sugar which could be a beer bomb disaster.