I always just mix a gallon at a time, which is 3lbs.HOH per gallon.
I you have a battery hydrometer, you can check the electrolyte level with that. It should read (if I remember correctly) around 1.275 for 28%. It won't work properly unless it's at least 1.225. Remember, as a drycell reactor, the electrolyte concentration is nowhere near as weak as for a wetcell.
280 grams is only .617lbs.. Get a quart of distilled water and add 340 grams. After it cools awhile, try it out. Your results should improve greatly.