Martin should probably answer this since it is his software, but a quick google turned up pure anhydrous citric acid on amazon, so yes yours is likely pure (not analytical grade, but pure enough).
When you are dealing with solutions made from powders, % refers to g/100mL, so 100% citric acid solution would be 100g in 100mL water (for solutions made from solutions, it is a little different). I have no idea what its solubility is. You should be able to enter whatever % you like in Bru'n Water, see how many mL it says to add, then do some math to convert that into mg or grams or anhydrous citric acid, and add that directly to your strike water, rather than making a stock solution. Or just make a stock solution.
I tend to take the path of least steps (i.e. just add powder to strike water).